"As a side note, in the case of Arc dropping these delimiters is a false economy because without parentheses around the (if ) clauses one must insert (do ) statements to include a sequence of S Expressions inside the (cond) clause."

IIRC before making this decision I analyzed a corpus of CL code and found that the great majority of cond clauses had only one expression after the test. I think the reason cond traditionally has implicit progn is simply that JMC did not initially separate the two concepts in his mind.

Incidentally, in Arc = is assignment.