I noticed that in Edwin (MIT Scheme 10.1.10), one can do C-M-i completion. For example, typing "(quoti" and then pressing C-M-i will complete the text to "(quotient", and typing "(strea" then pressing C-M-i will first complete to "(stream" and then pressing it again will give a list of all the builtin library commands like STREAM-PAIR?, STREAM-TRUNCATE, etc.
I also use MIT Scheme using Emacs 26, through (load-library 'xscheme) and then M-x start-scheme. The C-M-i feature described above appears to be missing. Is it really missing, or is it just a different command?