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Buy Prozac No Prescription, Pollard and Sag, in 1985, took the Generalized Phrase Structure Grammar model and pared it down, reducing the formalism to a more minimal set of tools in many ways reminiscent of the contemporaneous work in Government and Binding. Where GPSG had just arbitrary rules for production/constraints on local subtrees with arbitrary constraint equations, HPSG has a very small set of rules that license local subtrees with very specific constraints equations, and all variation comes not from having separate rules but from having different properties on the items in the subtree. HPSG was also very lexicalized, in that the general combinatory rules combine and manipulate features that come from the lexical items; nothing is contributed by the tree except the combination of these sets of features into new sets of features. And rather than employing normal constraint equations, HPSG looks more like LFG, in that it employs feature structures represented by AVMs.

Ontology of Feature Structures

The kinds of features and values found in feature structures vary by the particular version of HPSG you're looking at, but some fairly standard features of word and phrase feature structures are head, comps (for complements), Prozac recreational, and spec (for specifier). Phrasal feature structures also have daughters features (dtrs), sometimes broken into separate features for the different kinds of daughters (heads, comps, specs, adjuncts), sometimes just listed. Other features can include syntactic features located in a syn feature, semantic features in a sem (or the combination of both in a synsem feature), amongst other things, Buy Prozac No Prescription. The precise features are, overall, Prozac schedule, not significant enough to matter, and going from one variety of features to another is trivial.

Head-Complement Rule

The Head-Complement Rule is extraordinarily simple, and forms half of the HPSG-equivalent of Minimalist's Merge operation.

Much like Minimalism's Merge, the Head-Complement Rule says that a phrase with no requirements for its comps feature can be formed out of a word with some requirements followed by the things that satisfy those requirements, leaving its spec requirements untouched.

Head-Specifier Rule

Similarly for the Head-Specifier Rule we have

which says that a phrase with no specifier can be formed from a phrase (or word, presumably) that needs a specifier preceded by something that satisfies that requirement.

Head-Modifier Rule

And finally, for the core structure building rules of HPSG, we have the Head-Modifier Rule, which is subtly different than the other two rules. Rather than the head putting restrictions on the modifiers, like it does with the specifiers and complements it takes, japan, craiglist, ebay, overseas, paypal, the modifier has requirements of the head that it follows.

Unification

Unlike its predecessor GPSG, HPSG doesn't make wide use of constraint equations. While it would be true to say that it could make use of them, different methods are employed to achieve the same goal. The method is called unification, and a brief explanation is warranted given the above rules.

The idea of unification is roughly that of merging1. Buy Prozac No Prescription, Let's use ⊔ to denote unification. Unifying a number with itself, for instance, 1 ⊔ 1, Prozac without prescription, produces that very same number, which is sensible enough, and merging a number with any other number produces nothing at all. Unifying two sets, e.g. {1,2,3} ⊔ {4,5,6}, gives back the union of those two sets (hence the choice here of a symbol reminiscent of the symbol for set union), order Prozac online c.o.d. A more interesting idea is the unification of two feature structures. Unification of an empty feature structure with any other feature structure is just that other feature structure — simple and obvious enough, Buy Prozac No Prescription. Union of two non-empty feature structures is slightly trickier. If the to feature structures have different attribute names, then the unification of them is just the feature structure with all the attribute-value pairs of each. If they share a feature, then the respective values must unify. So for example:






That too is relatively simple and obvious. Buy Prozac No Prescription, But what about when the two feature structures share attributes with feature-structure values. In those cases, the values for the respective attributes must themselves unify to a feature structure (as opposed to unifying to nothing). What is Prozac, If any of the attributes of these sub-feature-structures (or attributes of sub-sub-feature-structures, etc.) fail to unify to something, then the two feature structures as a whole fail to unify to something. Hence in (2) the feature structures unify, while in (3) they do not. In (4) they also do not, but because of a failure to unify very deep inside the feature structures.








In HPSG, unification is the way of performing computations over structure. Extending the notion of unification slightly, in HPSG, when any two apparently distinct AVMs are marked by the same boxed symbol (usually a boxed number), this is taken to mean that these apparently distinct AVMs actually denote the same feature structure, Buy Prozac No Prescription. Thus, the feature structure that they both co-denote must be the unification of the feature structures that each AVM denotes alone. In this way, buy cheap Prozac, the co-marking of two AVMs serves to both require equality of feature structures that show up in different places, as well as provide a way to show only some of the features in the denoted feature structure. It is both an equation and a convenient notational device. Thus below in (5), while there are two different (sub-)AVMs marked with 1's, they actually denote the same feature structure, shown off to the right. Buy Prozac No Prescription, And in (6), tho there are two AVMs marked with 2's, they cannot unify (and so the two feature structures they're inside are inconsistent with one another).






What's important to note, tho, is that the co-marking of these AVMs does not imply that the unification is exactly the value denoted. Rather, these feature structures act as a sort of constraint — by co-marking two AVMs, Prozac used for, we say that whatever it is that they both denote must unify with what the AVMs by themselves. Thus feature structures and co-marked AVMs that denote them merely say what must hold of the values. In this way they are a shorthand for sets of constraint equations. It's also important to know that co-marking can be done on values other than AVMs, such as numbers or symbols, and here they just denote the primitive unificational concept of sameness.

Building a Simple Sentence

A number of perspectives can be taken on what trees (with AVMs for nodes) are actually for. The perspective I will take here is that trees are used to show the set of constraints on how the different feature structures of a tree's terminal nodes must relate, Buy Prozac No Prescription. As such, "building" is hardly the right word — a more accurate word might be "verifying" — tho one can imagine a program that takes two feature structures and puts them together in accordance to one of the rules given above, building up the larger tree, and, if it can successfully put all the relevant words together in the appropriate fashion, Prozac over the counter, it has built a tree identical to the one you would use to verify. In a sense, building is just piecemeal verification. However, for the sake of not forcing you to wade through more diagrams than necessary, we will look a tree from a purely verificational perspective.

To begin, we have a set of lexical entries that are by stipulation the feature structures for the words "the", "dog", "men", Prozac from canada, and "bites":




We can then look at a tree for the sentence "the dog bites the men" below, and we notice that all of the AVMs that are co-marked happen to be unifiable.

Working out way up the tree, we see that the feature structure for the and the feature structure for dog license a new feature structure for the subject using the Head-Specifier rule: the feature structure for the is marked with a 1, and the spec feature of dog is also marked with a 1, and the two feature structures unify because both have head cat equal to d. Thus the resulting feature structure, by the Head-Specifier rule, has an empty spec list, and the same comps (which is an empty list, in this case) and head as dog. Similar explanation holds for the and men.

Looking at the verb bites Buy Prozac No Prescription, now, we see that it has a single item in its comps feature, marked with a 5, which requires head cat to be n, and that the feature structure for the direct object (the result of putting the together with men) is also marked with a 5. Both feature structures unify, discount Prozac, because the direct object also has head comp equal to n, by way of sharing its head feature structure with that of the word men. Thus the verb together with the direct object license a new phrase by the Head-Complement rule, and the resulting phrase shares its head and spec features with the word bites.

Using the Head-Specific rule again we put together the verb phrase (the result of putting together the verb and the direct object) with the subject (the result of putting together the with dog) using familiar reasoning: the verb phrase's spec feature structure is marked with a 7, as is the subject's feature structure, they unify because their head features are the same.

The whole tree, which matches up with a sentence of English — "the dog bites the men" — unifies, and thus we have shown that this tree is a valid tree, and thus the sentence is grammatical. Prozac overnight, Notice, however, what happens if we swap the words dog and men around to try and produce the sentence "the men bites the dog": the subject's head feature structure does not match the verb phrase's spec feature structure because the verb phrase requires its spec to have head agr num equal to sg, while the subject has head agr num equal to pl. Because of this, the two cannot unify, and the tree is inadmissable, thus capturing the ungrammaticality of the sentence.

Long-distance Dependencies

Like GPSG before it, HPSG achieves long-distance dependencies by way of a feature that "absorbs" an argument of the verb (either a spec or a comps feature structure). In GPSG this was called slash, and depending on the version of HPSG it's either slash or gap, Buy Prozac No Prescription. We can write a rule for this gap-introduction process for a single comps like so:

Correspondingly, we have a rule for the gap-elimination process:

Notice that gaps can only be eliminated if the gap-possessing phrase has no more values in spec, Prozac street price. Real definitions of this vary, but this suffices to show demonstrate both topicalization and questions (omitting non-gapping-related parts of the feature-structures/trees):


As you can see, in both of these sentences we have "gapped" the direct object, allowing it to be fronted. The same mechanism thus allows us to handle two phenomena quite similarly

Relationship to Minimalism

The relationship between Chomskyan-style Minimalism (as opposed to some broader notion of linguistic minimalism) and Head-driven Phrase Structure Grammar is actually much tighter than it first seems. The feature sets of Minimalism are essentially very very simple kinds of feature structures — the phonological content of an item can be seen as equivalent to the phon feature of HPSG, the interpretable features of an item can be seen as a cat feature, and the uninterpretable features as subcat features or spec and comps features. Buy Prozac No Prescription, In Minimalism, uninterpretable features are peeled off as a head projects phrases by Merging with other items just as the spec and comp features are reduced to empty lists via the simple Head-Specificier and Head-Complement rules (don't ask about modification in Minimalism, noone's quite sure how to handle that. Heh.). Prozac mg, Minimalism's Move operation can be seen as analogous to single HPSG verification operation which takes a tree with no gaps and turns it into a tree with a complete acceptable gapping. The real big difference is the feature ontology: in Minimalism, features are very simple triples consisting of (in one way of defining them) a feature name, a + or - for interpretability, and a primitive symbol value, while in HPSG, features can very complex indeed, potentially including the whole feature structure of other words or phrases (as in the case of spec, comps, and gap). This affords HPSG simpler agreement mechanisms (subject-verb agreement is achieved simply by having the verb's spec include a feature structure with particular head agr values), Prozac steet value, but it also complicates the system by making it necessary to introduce computationally complex phenomena such as unification (whereas Minimalism requires at most set subtraction, a relatively simple process).


1 An alternative view of feature structure unification is that A ⊔ B denotes the smallest feature structure that contains both A and B as sub-structures. In the case where A and B have conflicting values, there is no such smallest feature structure, since features can only be valued once. In the case where there is no conflict, all is well, provided we understand "sub-structure" to be defined recursively: if both A and B have feature structures for the feature F, then A is a sub-structure of B if A's F feature is a sub-structure of B's F feature.

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