The method for combination depends if the two formula are one- or
two-sided.
If both formula are one-sided, the right hand side (RHS) of both are added
together with additional parentheses added, if parentheses appear to be
needed. If both formula are two-sided, the left hand side (LHS) and RHS are
separately added. If one formula is one-sided and the other is two-sided,
the LHS is selected from the two-sided formula and the RHS follows rules as
though two one-sided formula were added.
multiply_formula Multiply two formula (identical to (a~b) * (c~d)
divide_formula Divide two formula (identical to (a~b) / (c~d)
add_formula Add two formula (identical to (a~b) + (c~d)
subtract_formula Multiply two formula (identical to (a~b) - (c~d)
Ops.formula Supports generic binary operators and a couple of
unary operators (see ?Ops).
Math.formula Supports generic unary operators (see ?Math).