51. A computer-readable medium containing instructions for causing a computer system to navigate through selection modes for a displayed representation of computational construct that represents a terminal operand, the selection modes including a left selection mode, a tree selection mode, and a right selection mode, by: setting a current selection to a selection of the computational construct representing the terminal operand in the left selection mode so that, when a further computational construct representing a binary operator is specified, the terminal operand will be a right operand of the binary operator; when the current selection is the selection of the computational construct representing the terminal operand in the left selection mode and a user inputs a first indicator, setting the current selection to a selection of the computational construct representing the terminal operand in the tree selection mode so that, when a further computational construct is specified, the computational construct representing the terminal operand is replaced by the specified computational construct; and when the current selection is the selection of the computational construct representing the terminal operand in the tree selection mode and the user inputs the first indicator, setting the current selection to a selection of the computational construct representing the terminal operand in the right selection mode so that, when a further computational construct representing a binary operator is specified, the terminal operand will be a left operand of the binary operator.
51. A computer-readable medium containing instructions for causing a computer system to navigate through selection modes for a displayed representation of computational construct that represents a terminal operand, the selection modes including a left selection mode, a tree selection mode, and a right selection mode, by: setting a current selection to a selection of the computational construct representing the terminal operand in the left selection mode so that, when a further computational construct representing a binary operator is specified, the terminal operand will be a right operand of the binary operator; when the current selection is the selection of the computational construct representing the terminal operand in the left selection mode and a user inputs a first indicator, setting the current selection to a selection of the computational construct representing the terminal operand in the tree selection mode so that, when a further computational construct is specified, the computational construct representing the terminal operand is replaced by the specified computational construct; and when the current selection is the selection of the computational construct representing the terminal operand in the tree selection mode and the user inputs the first indicator, setting the current selection to a selection of the computational construct representing the terminal operand in the right selection mode so that, when a further computational construct representing a binary operator is specified, the terminal operand will be a left operand of the binary operator. 55. The computer-readable medium of claim 51, further comprising: when the current selection is the selection of the computational construct representing the terminal operand in the left selection mode and the user inputs a second indicator, setting the current selection to a selection of a parent computational construct of the computational construct representing the terminal operand in the tree selection mode.