When a function is evaluated, its arguments are evaluated individually in the order specified in the list of arguments, from left to right. The argument being evaluated can be a function-identifier, or it can be an expression containing function-identifiers.
If an arithmetic expression is specified as an argument, and if the first operator in the expression is a unary plus or a unary minus, it must be immediately preceded by a left parenthesis. For example, function MEAN(x-y z) would be the mean of two arguments: x-y and z.
MEAN((x) (-y) z)