# ElementaryFunctionSign(R, F)¶

sign(f, x, a, s) returns the sign of f as x nears a from below if s is “left”, or above if s is “right”.
sign(f, x, a) returns the sign of f as x nears a, from both sides if a is finite.
sign(f) returns 1 if f is always non-negative; returns -1 is f is always non-positive; returns 0 if f is always 0; returns “failed” if the sign of f varies or can't be determined.