Commonly used loop syntax is replete with syntax that breaks "op args ...":

  when <condition> do
  for <var> in <list>
  for <var0> = <init0> then <step0> and
    for <var1> = <init1> then <step1>
  collecting <expr> into <var>

The normal macros that don't have a flat argument structure, but shove everything into lists are keeping with the idea of leveraging the nesting so that the construct is "parsed on arrival" (POA?) into the macro. All that remains is to access its structure.