This is a parser for a very simple language, having expressions with four operators, assignment, variables and constants. The basic grammer is this:

Asst	→	Id:=Expr;
Expr	→	Expr(+|−)Term|Term
Term	→	Term(*|/)Fact|Fact
Fact	→	id|lit|(Expr)

But the parsing technique used, recursive descent, cannot tolerate left recursive rules. So we modify the grammer to:

Asst	→	Id:=Expr;
Expr	→	Term{(+|-)Term}
Term	→	Fact{(*|/)Fact}
Fact	→	id|lit|(Expr)

The idea is to build functions which match each left side symbol by trying to match the right-hand side. This program prints an abstract syntax for its input.