Ambiguity and Left Recursion
Many derivations could correspond to a single parse tree. A grammar for which some sentence has more than one parse tree is ambiguous. An ambiguous grammar can be dealt with in two ways:
A grammar is left recursive iff A +⇒ A α for some nonterminal A . A left recursive grammar can cause a top-down recursive descent parser to go into an infinite loop. Left recursion can be eliminated by left factoring to obtain an equivalent grammar.