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