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# How to Change Operator Definitions

We will illustrate with an infix operator and/2 and another or/2. We will choose the precedence of and/2 to be greater than that of or/2. This means that we interpret:

```
she is clever and rich or healthy

```
as

Since and/2 reminds us of ,/2 we will give it the same precedence and associativity:

The required command is

```
op(1000,xfy,and).

```
The predicate op/3 takes a precedence as its first argument, a legal associativity for its second argument and an operator name for its third argument. If given legal arguments, it succeeds with the side-effect of adding or changing an operator definition. You can even change the existing definitions ---but, be warned, this can be dangerous.

We could also make it like ,/2 by defining and/2 as in:

```
X and Y :-

call(X),

call(Y).

```
Note that we have to have defined and/2 as an operator before we can write the head of this clause as X and Y.

For or/2 we choose precedence of 950 (less than and/2) and associativity of xfy (the same as and/2) with:

```
op(950,xfy,or)

```
and define it as equivalent to:
```
X or Y :-

call(X).

X or Y :-

call(Y).

```

Paul Brna
Mon May 24 20:14:48 BST 1999