2. Consider the following grammar: btree NIL LPAREN btree COMMA value cOMMA btree RPAREN value digit | digit value digit : 1213141516171819 Some examples of binary trees are (NIL,12, NIL) NIL, 12, NIL),20, (NIL,15, NIL)) (NIL, 3,NIL),8,( (NIL, 12,NIL),20,(NIL, 15, NIL))) Augment this grammar with attributes and the attribute computation functions and predicates to accept binary trees that are "balanced". A balanced binary tree is one in which the heights of the subtrees at each interior node are within one of each other