Atomic Terms

These fall into four categories:

numerals:

0 and non-zero-initial strings of digits. Subsumed under constants for most purposes in this document; the main difference is that numerals are predeclared.

constants:

Strings consisting of characters from the set consisting of the letters (upper and lower case are allowed and distinct), digits, and the characters _ and ?. A numeral is not a constant, and a constant may not begin with ?. Numerals are subsumed under constants in most respects, except that they are predeclared.

free variables:

As constants, except that they begin with ? and should contain in addition some non-digit.

bound variables:

A nonzero numeral prefixed with a ? is a bound variable.