MSG is an utility.
MSG is the canonical way of producing text output, error or warning
messages etc. It has the general form (MSG item1 ... itemn) where each item
can be one of the following forms:
F -> (FRESH-LINE) ((TERPRI), but only of not at beginning of line)
THROW -> print (again using MSG) value of most recent THROW, usually THROWFAIL
(T n) -> (TAB n)
(TX n) -> (TABX n) (tabs without using <tab> characters)
(E form) -> evaluates form without printing the result
(L list) -> (PRINLC list) (print list without outermost parens)
(form . argtype) -> calls the printfn for argtype on form. This is extremely
useful for wffs, lines, type symbols etc.
n, n>0 -> (SPACES n)
n, n<0 -> -n times (TERPRI)
otherwise -> (PRINC otherwise)

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