Function classes

1. Introduction
2. Concepts
   1. NullaryFunctionClass?
   2. UnaryFunctionClass?
   3. BinaryFunctionClass?
   4. PredicateClass?
   5. BinaryPredicateClass?
3. Classes
   1. apply?
   2. Arithmetic operations
      1. plus?
      2. minus?
      3. multiplies?
      4. divides?
      5. modulus?
      6. negate?
   3. Comparisons
      1. equal_to?
      2. not_equal_to?
      3. less?
      4. greater?
      5. less_equal?
      6. greater_equal?
   4. Logical operations
      1. logical_or
      2. logical_and
      3. logical_not