Set operations on tables

The following operations are defined only when each operand shares the same exact table header.

t #= q ^ r // intersect
t #= q + r // union
t #= q - r // subtract (subset of q without r)
t #= q * r // multiply (cross product)
t #= q % r // symmetric difference (union minus intersection)
quantityOfR = ??t // total number of rows