Every Z-related command requires selection of its operands before the command can be applied.
A well-formed Z formula can be selected by
pointing the mouse and clicking button 1.
The selected formula is highlighted using inverse video,
and its syntactic category is named in the window's title bar.
If the formula thus selected is a smaller formula
within the desired one, click again within that smaller formula
to select the next larger formula; and repeat as necessary.
To select a large formula in few clicks,
click on a part of it that is involved in the fewest sub-formulae.
For example, to select a universal quantification predicate,
one click on the symbol or the
symbol suffices,
whereas a click within the schema text or predicate parts
would require one or more further clicks.
The majority of commands are unary, so require only one selection. Others require several selections to be made before the command can be applied. Many of the unary commands can be applied to several selections simultaneously (so long as they are applicable to every selection).
The way to make several selections for a single command is as follows. Make one selection, then hit the x key. This marks the selection by drawing a cross through it, and it ceases to be in inverse video. Repeat for the other selections, but omit crossing the last one: there has to be an inverse video selection for button 2 to pop-up the menu. This use of crosses allows all the selections to be perceived, even if they overlap. The c key erases the most recent cross (useful if a mistake has been made), and the C key clears all crosses at once.