[syntax] if3 c less equal greater

SRFI-67: If3 is the 3-way conditional for comparisons. First c is evaluated, resulting in value C. The value C must be an exact integer in {-1, 0, 1}, otherwise an error is signalled. If C = -1 then the value of the if3-expression is obtained by evaluating less. If C = 0 then equal is evaluated. If C = 1 then greater is evaluated.