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GNU Emacs Calc 2.02 Manual

18.5.6.6 Predicates

If the separator symbol is pred, the result of the formula is treated as a true/false value; calc-eval returns t or nil, respectively. A value is considered "true" if it is a non-zero number, or false if it is zero or if it is not a number.

For example, `(calc-eval "$<$$" 'pred a b)' tests whether one value is less than another.

As usual, it is also possible for calc-eval to return one of the error indicators described above. Lisp will interpret such an indicator as "true" if you don't check for it explicitly. If you wish to have an error register as "false", use something like `(eq (calc-eval ...) t)'.