The following is a simplified BNF description of the syntax of OPS5. Terminals are printed in a Roman type face, and non-terminals are printed in italics. The only nonstandard meta symbol used is the star (*). The star indicates that the preceding item is to be repeated zero or more times.
production ::= ( p constant-symbolic atom Ihs --> rhs )
lhs ::= positive-ce ce*
ce ::= positive-ce
::= negative-ce
positive-ce ::= form
::= { element-variable form }
::= { form element-variable }
negative-ce ::= - form
form ::= ( lhs-term* )
lhs-term ::= ^ constant-symbolic-atom Ihs-value
::= ^ number Ihs-value
::= lhs-value
lhs-value ::= { restriction* }
::= restriction
restriction ::= << any-atom* >>
::= predicate atomic-value
::= atomic-value
atomic-value ::= // any-atom
::= var-or-constant
var-or-constant ::= constant-symbolic-atom
::= number
::= variable
predicate ::= =
::= <>
::= <
::= <=
::= >=
::= >
::= <=>
rhs ::= action*
action ::= ( make rhs-term* )
::= ( remove element-designator* )
::= ( modify element-designator rhs-term*)
::= ( halt )
::= ( bind variable )
::= ( bind variable rhs-term* )
::= ( cbind element-variable )
::= ( call constant-symbolic-atom rhs-term* )
::= ( write rhs-tenn* )
::= ( openfile rhs-term* )
::= ( closefile rhs-term* )
::= ( default rhs-term* )
::= ( build quoted-form* )
element-designator ::= number
::= element-variable
rhs-term ::= ^ var-or-constant rhs-value
::= rhs-value
rhs-value ::= atomic-value
::= function
function ::= ( litval var-or-constant )
::= ( substr element-designator var-or-constant var-or-constant )
::= ( genatom )
::= ( crlf )
::= ( rjust var-or-constant )
::= ( tabto var-or-constant )
::= ( accept )
::= ( accept var-or-constant )
::= ( acceptline var-or-constant* )
::= ( compute expression )
::= user-defined-function
user-defined-function ::= ( constant-symbolic-atom var-or-constant* )
expression ::= number
::= variable
::= expression operator expression
::= ( expression )
operator ::= +
::= -
::= *
::= //
::= \\
quoted-form ::= \\ rhs-value
::= any-atom
::= ( quoted-form* )
Several terms have been left undefined: variable, element-variable, constant-symbolic-atom, any-atom, and number. Symbolic atoms and numbers are described in Section 2. The two kinds of variables are described in Sections 4 and 5. The only thing that needs to be explained here is the difference between any-atom and
constant-symbolic-atom. The former is an atom that is treated as a constant because it is quoted (with // or << >> usually). The latter is an atom that is. treated as a constant because it does not have the form of a variable or operator.