23. The apparatus of claim 1 and wherein: the objects and fields representing the mathematical expression are all contained in a root field; for every field there is a uniquely defined object-field sequence, the object-field sequence having: a first term which is an empty selection; a second term comprising the field itself; and if the field is not the root field a third term comprising the non-primitive object to which the field belongs; and the selection sequence continues with additional terms, alternating between fields and non-primitive objects containing them until the selection sequence terminates with the selection of the root field; and the user interface allows the user to optionally select one of more or less of the mathematical expression using respective select-more and select-less functions, and wherein: the select-more function selects the smallest term of a first subsequence of the object-field sequence that contains a current selection but is not equal to it; and the select-less function selects the largest term in a second subsequence of the object-field sequence that is contained by the current selection but is not equal to it.
23. The apparatus of claim 1 and wherein: the objects and fields representing the mathematical expression are all contained in a root field; for every field there is a uniquely defined object-field sequence, the object-field sequence having: a first term which is an empty selection; a second term comprising the field itself; and if the field is not the root field a third term comprising the non-primitive object to which the field belongs; and the selection sequence continues with additional terms, alternating between fields and non-primitive objects containing them until the selection sequence terminates with the selection of the root field; and the user interface allows the user to optionally select one of more or less of the mathematical expression using respective select-more and select-less functions, and wherein: the select-more function selects the smallest term of a first subsequence of the object-field sequence that contains a current selection but is not equal to it; and the select-less function selects the largest term in a second subsequence of the object-field sequence that is contained by the current selection but is not equal to it. 25. The apparatus of claim 23 and wherein the user interface comprises a pointing device having at least one pointing device control, and the series of computer-executable instructions comprise a series of steps to allow the user to operate the select-more function by repeated engagement of the pointing device control.