truncate toward zero (C):          -3  remainder -1.0
floor (python //, our FLOORDIV):   -4  remainder 1
our floordiv/floormod on (-7, 2):  -4 1
python 7 // -2 = -4   7 % -2 = -1  (the remainder takes the sign of the divisor)
identity a == b*(a//b) + a%b over a in [-20,20], b in [-5,5], b!=0: True
x//0 and x%0 under our contract: 0 7   INT_MIN // -1: -2147483648
