To install this package, run in Emacs:
M-x package-install RET gnu-apl-mode RET
Emacs mode for GNU APL This mode provides both normal editing facilities for APL code as well as an interactive mode. The interactive mode is started using the command ‘gnu-apl’. The mode provides two different ways to input APL symbols. The first method is enabled by default, and simply binds keys with the "super" modifier. The problem with this method is that the "super" modifier has to be enabled, and any shortcuts added by the operating system that uses this key has to be changed. The other method is a bit more cumbersome to use, but it's pretty much guaranteed to work everywhere. Simply enable the input mode using C-\ (‘toggle-input-method’) and choose APL-Z. Once this mode is enabled, press "." (period) followed by a letter to generate the corresponding symbol.