When an object O is being considered iteratively
against each element E_i
of a sequence S
by an operator F listed in Figure 17–2,
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 |
The object O might not be compared directly to E_i.
If a :key argument is provided,
it is a designator for a function of one argument
to be called with each E_i as an argument,
and yielding an object Z_i to be used for comparison.
(If there is no :key argument, Z_i is E_i.)
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 Z_i.
An E_i is said (or, sometimes, an O and an E_i 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 Z_i.
An E_i is said (or, sometimes, an O and an E_i 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.
| Examples of Satisfying a Two-Argument Test |