description:
mbr(ket, sp) returns the coeff of 'ket' in 'sp'.
If 'ket' not in 'sp' then return |>
examples:
mbr(|b>, |a> + |b> + |c>)
|b>
mbr(|c>, 0.3|a> + 2|b> + 9.7|c> + 13|d>)
9.7|c>
mbr(|x>, 0.3|a> + 2|b> + 9.7|c> + 13|d>)
|>
see also:
is-mbr, intersection