#### An Example

As an example, here is a simple grammar that parses an arithmetic expression (made up of digits and operators) and computes its value. Create a file containing the following rules:

```                                 grammar.pl

expr(Z) --> term(X), "+", expr(Y), {Z is X + Y}.
expr(Z) --> term(X), "-", expr(Y), {Z is X - Y}.
expr(X) --> term(X).

term(Z) --> number(X), "*", term(Y), {Z is X * Y}.
term(Z) --> number(X), "/", term(Y), {Z is X / Y}.
term(Z) --> number(Z).

number(C) --> "+", number(C).
number(C) --> "-", number(X), {C is -X}.
number(X) --> [C], {"0"=<C, C=<"9", X is C - "0"}.
```

In the last rule, C is the ASCII code of a decimal digit.

This grammar can now be used to parse and evaluate an expression by means of the built-in predicates `phrase/2` and `phrase/3`. For example,

```     | ?- [grammar].
| ?-  phrase(expr(Z), "-2+3*5+1").

Z = 14

| ?-  phrase(expr(Z), "-2+3*5", Rest).

Z = 13,
Rest = [] ;

Z = 1,
Rest = "*5" ;

Z = -2,
Rest = "+3*5" ;

no
```