To select a sub-formula, move the Emacs cursor to any character in that
sub-formula, and press `j s` (`calc-select-here`

). Calc will
highlight the smallest portion of the formula that contains that
character. By default the sub-formula is highlighted by blanking out
all of the rest of the formula with dots. Selection works in any
display mode but is perhaps easiest in "big" (`d B`) mode.
Suppose you enter the following formula:

3 ___ (a + b) + V c 1: --------------- 2 x + 1

(by typing `' ((a+b)^3 + sqrt(c)) / (2x+1)`). If you move the
cursor to the letter ``b'` and press `j s`, the display changes
to

. ... .. . b. . . . 1* ............... . . . .

Every character not part of the sub-formula ``b'` has been changed
to a dot. The ``*'` next to the line number is to remind you that
the formula has a portion of it selected. (In this case, it's very
obvious, but it might not always be. If Embedded Mode is enabled,
the word ``Sel'` also appears in the mode line because the stack
may not be visible. see section Embedded Mode.)

If you had instead placed the cursor on the parenthesis immediately to
the right of the ``b'`, the selection would have been:

. ... (a + b) . . . 1* ............... . . . .

The portion selected is always large enough to be considered a complete
formula all by itself, so selecting the parenthesis selects the whole
formula that it encloses. Putting the cursor on the the ``+'` sign
would have had the same effect.

(Strictly speaking, the Emacs cursor is really the manifestation of
the Emacs "point," which is a position *between* two characters
in the buffer. So purists would say that Calc selects the smallest
sub-formula which contains the character to the right of "point.")

If you supply a numeric prefix argument `n`, the selection is
expanded to the `n`th enclosing sub-formula. Thus, positioning
the cursor on the ``b'` and typing `C-u 1 j s` will select
``a + b'`; typing `C-u 2 j s` will select ``(a + b)^3'`,
and so on.

If the cursor is not on any part of the formula, or if you give a numeric prefix that is too large, the entire formula is selected.

If the cursor is on the ``.'` line that marks the top of the stack
(i.e., its normal "rest position"), this command selects the entire
formula at stack level 1. Most selection commands similarly operate
on the formula at the top of the stack if you haven't positioned the
cursor on any stack entry.

The `j a` (`calc-select-additional`

) command enlarges the
current selection to encompass the cursor. To select the smallest
sub-formula defined by two different points, move to the first and
press `j s`, then move to the other and press `j a`. This
is roughly analogous to using `C-@` (`set-mark-command`

) to
select the two ends of a region of text during normal Emacs editing.

The `j o` (`calc-select-once`

) command selects a formula in
exactly the same way as `j s`, except that the selection will
last only as long as the next command that uses it. For example,
`j o 1 +` is a handy way to add one to the sub-formula indicated
by the cursor.

(A somewhat more precise definition: The `j o` command sets a flag
such that the next command involving selected stack entries will clear
the selections on those stack entries afterwards. All other selection
commands except `j a` and `j O` clear this flag.)

The `j S` (`calc-select-here-maybe`

) and `j O`
(`calc-select-once-maybe`

) commands are equivalent to `j s`
and `j o`, respectively, except that if the formula already
has a selection they have no effect. This is analogous to the
behavior of some commands such as `j r` (`calc-rewrite-selection`

;
see section Selections with Rewrite Rules) and is mainly intended to be
used in keyboard macros that implement your own selection-oriented
commands.

Selection of sub-formulas normally treats associative terms like
``a + b - c + d'` and ``x * y * z'` as single levels of the formula.
If you place the cursor anywhere inside ``a + b - c + d'` except
on one of the variable names and use `j s`, you will select the
entire four-term sum.

The `j b` (`calc-break-selections`

) command controls a mode
in which the "deep structure" of these associative formulas shows
through. Calc actually stores the above formulas as ``((a + b) - c) + d'`
and ``x * (y * z)'`. (Note that for certain obscure reasons, Calc
treats multiplication as right-associative.) Once you have enabled
`j b` mode, selecting with the cursor on the ``-'` sign would
only select the ``a + b - c'` portion, which makes sense when the
deep structure of the sum is considered. There is no way to select
the ``b - c + d'` portion; although this might initially look
like just as legitimate a sub-formula as ``a + b - c'`, the deep
structure shows that it isn't. The `d U` command can be used
to view the deep structure of any formula (see section Normal Language Modes).

When `j b` mode has not been enabled, the deep structure is
generally hidden by the selection commands--what you see is what
you get.

The `j u` (`calc-unselect`

) command unselects the formula
that the cursor is on. If there was no selection in the formula,
this command has no effect. With a numeric prefix argument, it
unselects the `n`th stack element rather than using the cursor
position.

The `j c` (`calc-clear-selections`

) command unselects all
stack elements.

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