code: def primary_spin(mass1, mass2, spin1, spin2):
      
      mass1, mass2, spin1, spin2, input_is_array = ensurearray(
          mass1, mass2, spin1, spin2)
      sp = copy.copy(spin1)
      mask = mass1 < mass2
      sp[mask] = spin2[mask]
      return formatreturn(sp, input_is_array)
docs: Returns the dimensionless spin of the primary mass.