“Procedures returning procedures” is an important idiom of Motto. Consider a simple procedure called complement that takes a predicate p and returns its inverse. The following code snippet demonstrates how complement could be used:

> [2 remainder 0 eq] def is-even
> 10 is-even
#t
> 3 is-even
#f
> @is-even complement def is-odd
> 3 is-odd
#t
> 2 is-odd
#f

Here we used complement to produce a new procedure which gives exactly the opposite result of is-even. complement itself is defined as:

> [def p [p not]] def complement

Another example:

> [def vowel a e i o u list vowel swap list-contains] def is-vowel
> @is-vowel complement def is-consonant
> a is-vowel
#t
> b is-vowel
#f
> b is-consonant
#t

Note: The value of a procedure can be extracted by preceding its name with an @. This is an alternative to value procedure-name.

Reference: On Lisp.