In formal language theory, the left corner of a production rule in a context-free grammar is the left-most symbol on the right side of the rule.9.3 Using Left-corner Tables, Patrick Blackburn and Kristina Striegnitz, Natural Language Processing Techniques in Prolog
For example, in the rule A→Xα, X is the left corner.
The left corner table associates to a symbol all possible left corners for that symbol, and the left corners of those symbols, etc.
Given the grammar S → VP S → NP VP VP → V NP NP → DET N
the left corner table is as follows.
SymbolLeft corner(s)SVP, NP, V, DETNPDETVPV
Left corners are used to add bottom-up filtering to a top-down parser, or top-down filtering to a bottom-up parser.
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