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.