Unindents selected source code.
a+ and a- use your indentation settings to indent or unindent
your selection in acme using either tabs or spaces depending on
what is configured. To use these commands, write |a+ or |a- to
the scratch area in your acme window, select the text you want to
indent, and then middle click on |a+ to indent or |a- to unindent
your selection. These commands are also used by nynetab for
indenting/unindenting text depending on the provided flags.