Making Selections
-----------------

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 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 Nth 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 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 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 Nth
stack element rather than using the cursor position.

The `j c' (`calc-clear-selections') command unselects all stack
elements.