The shorthands are defined in ly/script-init.ly, where the
variables dashHat, dashPlus, dashDash,
dashBang, dashLarger, dashDot, and
dashUnderscore are assigned default values. The default values
for the shorthands can be modified. For example, to associate the
-+ (dashPlus) shorthand with the trill symbol
instead of the default + symbol, assign the value \trill
to the variable dashPlus: