To save space, optmatch objects merely store a hash of the distance matrix
instead of the original object. This checks if the hash of
identical to the hash currently saved in
An optmatch object.
Note that the distance is hashed with its
call set to
NULL. (This avoids issues where, for example,
data=d, caliper=NULL) and
match_on(Z~X, data=d) produce identical
matches (since the default argument to
Boolean whether the two distance specifications are identical.