formula.substituting_formula
                        Convert a substituting_formula object into a
                        regular formula.
formula_side            Extract formula parts
modify_formula          Modify a formula by finding some part of it and
                        replacing it with a new value.
op_formula              Perform a mathematical operation on two formula
simplify_parens         Remove extraneous parentheses from a formula.
substituting_formula    A substituting formula helps clarify a formula
                        where the parameters are more simply described
                        in separate formulae.
