16. What is the FIRST set for a given grammar and sentential form?
Answer :
FIRST( ) = {a => * a } (If => * , is in FIRST( ))
in which =>* means 0 or more derivation steps
17. Describe the pairwise disjointness test.

Answer : 
It is a test of non-left recursive grammar that indicates whether left recursion can be done. It requires the ability to compute a set based on the RHS of a given nonterminal symbol in a grammar.
18. What is left factoring ?
Answer : 
Left factoring is the action taken when a grammar leads backtracking while marking parsing.syntax tree.
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19. What is a phrase of a sentential form ?
Answer :
A phrase is a subsequence of a sentential form that is eventually reduced to a single non-termin
20. What is a simple phrases of a sentential form ?
Answer : 
Simple phrases is just a phrases that takes a single derivation step from it’s root non-terminal node.