The GNU APL's editor is a clone of APL2's line editor, and thus
should support the same commands:
Command Action
[⎕] display the current whole definition
[⎕2 7 1 …] display specific lines
[⎕2-7] display a range of lines
[⎕-7] display all lines until a specific line
[⎕7-] display all lines beginning with a specific line
[7] code replace a specific line
[2.1] code insert a line between two lines
[∆2 7 1] delete specific lines
[∆2-7] delete a range of lines
[∆-7] delete all lines until a specific line
[∆7-] delete all lines beginning with a specific line
[→] quit without saving
[2⎕7] display line 2 for editing, placing cursor at pos 7
[2⎕0] display line 2 for editing, placing cursor at EOL
APL 2 Programming: Language Reference (Ch. 9, 'The APL 2 Editors')