58. The computer-readable medium of claim 57 wherein the first indicator is a tab and the second indicator is a shift-tab.
57. The computer-readable medium of claim 55, further comprising: when the current selection is the selection of the computational construct representing the terminal operand in the right selection mode and the user inputs a second indicator, setting the current selection to the selection of a parent computational construct of the computational construct representing the terminal operand in the tree selection mode.