The package performs two functions: to make sub- and superscript operations macros, which has the effect of making their behaviour uniform and more readily understandable; and to take advantage of the lack of infix operations (such as \over) in LaTeX to redesign the control of mathematical typesetting style, and to rationalise the way \mathchoice and dependent mechanisms work. The package offers support for breqn and is part of the bundle of the same name.