All of Guile's dedicated association list procedures, apart from
acons, come in three flavours, depending on the level of equality
that is required to decide whether an existing key in the association
list is the same as the key that the procedure call uses to identify the
eq?to determine key equality.
eqv?to determine key equality.
equal?to determine key equality.
acons is an exception because it is used to build association
lists which do not require their entries' keys to be unique.