Atomic terms

Atomic terms of the language of Watson are of four kinds. All atomic terms are represented by strings of nonzero length of alphanumeric characters (Watson is case-sensitive) plus the special characters ? and _.

numerals:

A string consisting entirely of digits is a numeral.

bound variables:

A string consisting of a single ? followed by a string of digits of nonzero length is a bound variable.

free variables:

A string beginning with ? which is not a bound variable is a free variable (? by itself is a free variable).

constants:

A string beginning with a character other than ? and containing at least one non-digit is a constant.