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### 6.3 Trail Commands

The commands for manipulating the Calc Trail buffer are two-key sequences beginning with the `t` prefix.

The `t d` (`calc-trail-display`) command turns display of the trail on and off. Normally the trail display is toggled on if it was off, off if it was on. With a numeric prefix of zero, this command always turns the trail off; with a prefix of one, it always turns the trail on. The other trail-manipulation commands described here automatically turn the trail on. Note that when the trail is off values are still recorded there; they are simply not displayed. To set Emacs to turn the trail off by default, type `t d` and then save the mode settings with `m m` (`calc-save-modes`).

The `t i` (`calc-trail-in`) and `t o` (`calc-trail-out`) commands switch the cursor into and out of the Calc Trail window. In practice they are rarely used, since the commands shown below are a more convenient way to move around in the trail, and they work “by remote control” when the cursor is still in the Calculator window.

There is a trail pointer which selects some entry of the trail at any given time. The trail pointer looks like a ‘`>`’ symbol right before the selected number. The following commands operate on the trail pointer in various ways.

The `t y` (`calc-trail-yank`) command reads the selected value in the trail and pushes it onto the Calculator stack. It allows you to re-use any previously computed value without retyping. With a numeric prefix argument n, it yanks the value n lines above the current trail pointer.

The `t <` (`calc-trail-scroll-left`) and `t >` (`calc-trail-scroll-right`) commands horizontally scroll the trail window left or right by one half of its width.

The `t n` (`calc-trail-next`) and `t p` (`calc-trail-previous)` commands move the trail pointer down or up one line. The `t f` (`calc-trail-forward`) and `t b` (`calc-trail-backward`) commands move the trail pointer down or up one screenful at a time. All of these commands accept numeric prefix arguments to move several lines or screenfuls at a time.

The `t [` (`calc-trail-first`) and `t ]` (`calc-trail-last`) commands move the trail pointer to the first or last line of the trail. The `t h` (`calc-trail-here`) command moves the trail pointer to the cursor position; unlike the other trail commands, `t h` works only when Calc Trail is the selected window.

The `t s` (`calc-trail-isearch-forward`) and `t r` (`calc-trail-isearch-backward`) commands perform an incremental search forward or backward through the trail. You can press <RET> to terminate the search; the trail pointer moves to the current line. If you cancel the search with `C-g`, the trail pointer stays where it was when the search began.

The `t m` (`calc-trail-marker`) command allows you to enter a line of text of your own choosing into the trail. The text is inserted after the line containing the trail pointer; this usually means it is added to the end of the trail. Trail markers are useful mainly as the targets for later incremental searches in the trail.

The `t k` (`calc-trail-kill`) command removes the selected line from the trail. The line is saved in the Emacs kill ring suitable for yanking into another buffer, but it is not easy to yank the text back into the trail buffer. With a numeric prefix argument, this command kills the n lines below or above the selected one.

The `t .` (`calc-full-trail-vectors`) command is described elsewhere; see Vector and Matrix Formats.