Searches a list of pairs for a pair whose first component equals a specified
value.
DESCRIPTION
assoc x [(x1,y1),...,(xn,yn)] returns the first (xi,yi) in the list such
that xi equals x. The lookup is done on an eqtype, i.e., the SML
implementation must be able to decide equality for the type of x.
FAILURE
Fails if no matching pair is found. This will always be the case if the list
is empty.