mathematical ternary operator: not-greater-prime-counting function

Outputs the number of prime numbers that are less than or equal to the input value a according to ordering rules b in ring c; contextless default for c is the standard/typical ring of integers (Z,+,*); contextless default for b is the standard typical ordering of the ring, which is the 'normal' absolute value (signless difference/distance with/from 0) for the standard ring (Z,+,*); thus, it will usually operate as an unary operator. Usually denoted "π(n)" in the literature.