ket1 %% ket2
return the value of ket1 modulus the value of ket2
we do our best to preserve categories
if one ket has a category, and the other doesn't then the result has that category
if ket1 and ket2 have different categories then return the empty ket |>
if ket1 and ket2 are 2D, then apply modulus component-wise
NB: the values for ket1 and ket2 must be integers