Parsing the sentence: (|a| |a| |b| |b|) PARSE STACK INPUT STACK ACTION (0) (|a| |a| |b| |b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1) (|a| |a| |b| |b| $) (SHIFT 2) (0 S 1 |a| 2) (|a| |b| |b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1 |a| 2 S 3) (|a| |b| |b| $) (SHIFT 2) (0 S 1 |a| 2 S 3 |a| 2) (|b| |b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1 |a| 2 S 3 |a| 2 S 3) (|b| |b| $) (SHIFT 5) (0 S 1 |a| 2 S 3 |a| 2 S 3 |b| 5) (|b| $) (REDUCE 1 (S -> S |a| S |b|)) (0 S 1 |a| 2 S 3) (|b| $) (SHIFT 5) (0 S 1 |a| 2 S 3 |b| 5) ($) (REDUCE 1 (S -> S |a| S |b|)) (0 S 1) ($) (ACCEPT) Sentence was grammatical. Parsing the sentence: (|a| |a| |b|) PARSE STACK INPUT STACK ACTION (0) (|a| |a| |b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1) (|a| |a| |b| $) (SHIFT 2) (0 S 1 |a| 2) (|a| |b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1 |a| 2 S 3) (|a| |b| $) (SHIFT 2) (0 S 1 |a| 2 S 3 |a| 2) (|b| $) (REDUCE 2 (S -> EPSILON)) (0 S 1 |a| 2 S 3 |a| 2 S 3) (|b| $) (SHIFT 5) (0 S 1 |a| 2 S 3 |a| 2 S 3 |b| 5) ($) (REDUCE 1 (S -> S |a| S |b|)) (0 S 1 |a| 2 S 3) ($) ERROR error - expecting one of the symbols |a| |b| Sentence was not in the grammar.