unary mekso operator: parity of function; if the input is a unary real-valued function X1 which is defined on a subset of the reals, then the output is 1 is X1 is even, -1 if X1 is odd, and 0 otherwise.
Let f be a unary function which maps a subset of the reals to the reals; then f is even iff f(-x) = f(x) for all x which belong to the domain of f, and f is odd iff f(-x) = -f(x) for all x which belong to the domain of f; for such f, it is possible for f to be both even and odd (in which case it maps valid inputs uniformly to 0) and it is possible for f to be neither even nor odd (even if its domain is symmetric over 0). For other types of functions, this word may be defined, so long as the definition is consistent with restrictions to the aforespecified domain and codomain. See also: "mai'u".