mekso operator: part of number/projection (one sense); the X2 part of X1

X1 can be a tensor, in which case the operator applies entry-wise; X1 can be a function, in which case the operator applies pointwise. It extracts the part of the number that belongs to the structure X2 united with the singleton 0. X2 = R implies "real part"; X2 = i*R implies "imaginary part"; X2 = R+ implies "positive part"; X2 = R- implies "negatige part". If X1 has no explicit value/projection in/along X2, then the output is 0. If 0 is X1 or X2, then the output is 0. X1 should really be a structure, not just a set.