Aquamacs: why does M-p suddenly start entering pi?

Question

Aquamacs 3.3 GNU Emacs 25.1.1 (x86_64-apple-darwin14.1.0, NS appkit-1344.72 Version 10.10.2 (Build 14C109)) of 2016-09-19 on 24a02dbf6b34ae061ef4df89f15bfbc5d3ed497e

OSX 10.10.5

Typically, when I'm in an erlang shell I can use M-p to cycle through previous commands, but occasionally when I do that the pi symbol gets entered, and I can't get the emacs functionality back. After that, M-p enters pi no matter what type of buffer I am in. I thought I had a fix: quit Aquamacs, but now that doesn't work anymore.

And, why is there no autocompletion of function names in the erlang shell--like with an erlang shell running in a Terminal window?

emacs is soooo frustrating!

Answer 1

I found the answer at Oliver Müller's blog. Somehow, in the course of (mis)typing emacs key sequences, I ended up with this (see the Aquamacs menu bar) :

Options > 
     Option, Command, Meta keys >
          ✓Option for composed characters
           Option is Meta

Simply clicking Option is Meta returns Aquamacs to its default state, allowing me to use the Option key as the Meta key again for emacs key sequences.

Note: in short order some key sequence I typed changed the Option key back to Option for composed characters.

Edit: It looks like C-; might be the offending key sequence. In any case, C-; can be used to toggle Option is Meta on and off.