Mathematical operators take one or more operands, which are here referred to by an 'x' subscripted with a number.
Operators defined here as "unary" must have no more than one operand that is not equal to tu'o (referred to simply as "the operand" within the respective definitions) and which may occur at any position in the operator's operand list.
See also: CLL section 18.16, example 16.3 and the paragraph before it, in which the operand of va'a is in the x_1 place; CLL section 18.14, example 14.1, in which the operand of va'a is in the x_2 place.
If all operands of an operator are tu'o, the value of the expression is equal to tu'o.
When an expression is here said to be "undefined," it means that, in the absence of external information about the interpretation of the expression, it is semantically void, meaningless, and/or nonsensical.