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### 2.7.1 A New Operator

With regard to a set of trees as described in Raw Material, we can define a new binary operator. Unlike the `cons` operator, this one is not required to associate an element of the set with every possible pair of elements. For very many pairs of operands we will have nothing to say about its result. In fact, we require nothing of it beyond a few simple properties to be described presently.

Because this is the only other operator than `cons`, there is no need to have a special notation for it, so it will be denoted by empty space. The tree associated by the operator with a pair of trees `x` and `y`, if any, will be expressed in the infix notation `x y`. For convenience, the operator is regarded as being right associative, so that `a b c` can be written for `a (b c)`.

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