description:
subset(one, two) returns the degree of subsetness of one with respect to two
where one and two are superpositions
examples:
subset(|b>, |a> + |b> + |c>)
|subset>
subset(|c>, 0.3|a> + 2|b> + 9.7|c> + 13|d>)
|subset>
subset(|b> + |d>, |a> + |b> + |c>)
0.5|subset>
see also:
is-subset, mbr, is-mbr