CLHS: Section 17.2.1

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## 17.2.1 Satisfying a Two-Argument Test

When an object O is being considered iteratively against each element Ei of a sequence S by an operator F listed in the next figure, it is sometimes useful to control the way in which the presence of O is tested in S is tested by F. This control is offered on the basis of a function designated with either a `:test` or `:test-not` argument.

    adjoin           nset-exclusive-or  search
    assoc            nsublis            set-difference
    count            nsubst             set-exclusive-or
    delete           nsubstitute        sublis
    find             nunion             subsetp
    intersection     position           subst
    member           pushnew            substitute
    mismatch         rassoc             tree-equal
    nintersection    remove             union
    nset-difference  remove-duplicates

Figure 17-2. Operators that have Two-Argument Tests to be Satisfied

The object O might not be compared directly to Ei. If a `:key` argument is provided, it is a designator for a function of one argument to be called with each Ei as an argument, and yielding an object Zi to be used for comparison. (If there is no `:key` argument, Zi is Ei.)

The function designated by the `:key` argument is never called on O itself. However, if the function operates on multiple sequences (e.g., as happens in set-difference), O will be the result of calling the `:key` function on an element of the other sequence.

A `:test` argument, if supplied to F, is a designator for a function of two arguments, O and Zi. An Ei is said (or, sometimes, an O and an Ei are said) to satisfy the test if this `:test` function returns a generalized boolean representing true.

A `:test-not` argument, if supplied to F, is designator for a function of two arguments, O and Zi. An Ei is said (or, sometimes, an O and an Ei are said) to satisfy the test if this `:test-not` function returns a generalized boolean representing false.

If neither a `:test` nor a `:test-not` argument is supplied, it is as if a `:test` argument of `#'eql` was supplied.

The consequences are unspecified if both a `:test` and a `:test-not` argument are supplied in the same call to F.

## 17.2.1.1 Examples of Satisfying a Two-Argument Test

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