A function or function object that returns a value testable as true
Negators, unary_negate, binary_negate, Operators, greater, less, greater_equal, less_equal, logical_and, logical_or, logical_not, binary_function, unary_function, plus, minus, multiplies, divides, modulus
ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25 -7-