An abstract syntax tree (AST) is a finite , labeled, directed tree, where the nodes are labeled by operators, and the edges represent the operands of the node operators. Thus, the leaves have nullary operators, i.e., pointers to the symbol table entries of the variables or constants. An AST differs from a parse tree by omitting nodes and edges for syntax rules that do not affect the semantics of the program. The classic example of such an omission is grouping parentheses, since in an AST the grouping of operands is explicit in the tree structure.