3.3.7 Select

Select causes the subject to interact with a collection of expressions called the selection set. The selection set is used by Gather and thus indirectly supports Linear and Solve . The selection set also provides expressions to the plotter (see §7).

Subexpressions in the selection set are indicated by highlighting. They can be scattered throughout the expressions on the algebra display. There can even be more than one subexpression selected into the set from the same expression. When Select is applied to a subject already in the selection set, the subject is removed. A subexpression is also removed if it is changed by some other transformation or if the expression of which it is a part is removed.

The selection set is cleared by . This interaction is somewhat counterintuitive because select-all collects all expressions and remains in effect until a new active expression is selected or until Select is applied. At this point, the selection set is definitely cleared. The selection set is also cleared by actions that use it.

Expressions can be added to the selection set using shift-click. This technique can only be used to add entire expressions to the selection set. If subexpressions are required, select them and use explicit Select .

Shift click is simulated by a sequential double touch. The first touch can be anywhere in the workspace that will not produce an unexpected change in selection. The second touch (while the first touch position is held) should be on an expression. The entire expression is added to the selection set.