I've seen EQUAL? in some of the code snippets. I can believe that inspired Scheme predicates.

Yes. But I think that's relatively minor.

From 'The Evolution of Lisp' Steele/Gabriel:

> MDL also introduced through example the use of a final ? in a name to connote a predicate ... Lisp Machine Lisp would pick up the lambda lists and the locatives, and would adapt the evaluation and splicing markers in backquote, though the evolution of the concept passes through Conniver where it is refined into a form very similar in appearance and effect as in Lisp-Machine Lisp, but in fact the implementation is quite different. Scheme would pick up the use of a trailing “?” for predicates. MDL had a multitasking facility that would reappear in various forms in Lisp-Machine Lisp, Conniver, and various implementations of Common Lisp.

Scheme got the '?'.

LML got complex arglists, locatives, markers for evaluation/splicing in backquote, multitasking.

Hmm, which evaluation and splicing markers is it referring to?

MDL doesn't have quasiquoting, but it has "segments" that expanded while evaluating a structure (prefix a form with "!", and the result is spliced into the structure), and reader macros expanded while parsing source code (prefix a form with "%" and the result is inserted, or "%%" and it's discarded).

Yeah, MDL used the question mark suffix like Lisp used "p".