Compound terms are of four kinds, syntactically. The deep structure of certain terms is not the same as the surface syntax. The description given here is appropriate to the default syntax of Watson; we will indicate below how this is modified by user-defined precedence and grouping, which is available as an option in Watson, though it has not been used so far in any extensive theory development.

**prefix terms:**- An operator followed by a term is a term. As will be described below,
some surface prefix terms are
actually infix terms in disguise.
**parenthesized terms:**- A term enclosed in parentheses
`()`is a term. Of course, a parenthesized term does not differ in any way in its internal representation from the same term without parentheses. **abstraction terms:**- A term enclosed in brackets
`[]`is a term. **infix terms:**- An atomic, parenthesized or function term followed
by an operator followed by a term is a term.
**case expression restriction:**- The only terms of the form
`term1 || term2`permitted are of the form`term1 || term2 , term3`. The apparent subterm`term2 , term3`of such a term does not correspond to anything in the internal representation of such a term (i.e.,`__ || __ , __`is a 3-place mixfix operator).