Sequence functions and circular arguments

[Nikodemus Siivola]

  COERCE, MERGE and CONCATENATE go into an infinite loop when given
  circular arguments; it would be good for the user if they could be
  given an error instead (ANSI 17.1.1 allows this behaviour on the part
  of the implementation, as conforming code cannot give non-proper
  sequences to these functions. MAP also has this problem (and
  solution), though arguably the convenience of being able to do
    (MAP 'LIST '+ FOO '#1=(1 . #1#))
  might be classed as more important (though signalling an error when
  all of the arguments are circular is probably desireable).

[jobach (form-t)]

REDUCE has the same problem, even if one gives end explicit value for :END (say n).
Mayby it might be reasonable to to process n elements (instead of signaling an error or looping infinitly)
when :END is given.