mekso 4-nary operator: spherical harmonics on colatitudinal/polar angle a and azimuthal/longitudinal angle b of unassociated order c and associated order d.

Usually denoted Y^m_{l (\theta, \phi)}. The Condon-Shortley phase must be prepended to the definition. The normalization is chosen so that the integral over all (solid) angles of Y^m_{l(\Omega) conj(Y^n}= \delta(m,n) \delta(l,k).