Every invocation is a (list of) comparand(s separated by a comparator operator). A comparand is a (list of) term(s separated by adder operators). A term is a (list of) factor(s separated by multiplier operators). A factor is a (list of) reference(s separated by power operators). Hence these four rules merely implement the precendence rules of the Pico concrete grammar. In the end, everything is just a basic building block, called a reference.