Why would one look for an invalid case that shouldn't be there in
the first place? That would be an unnecessary operation.
if the 1st character is '.' and the 2nd character is not end of line,
remove the first character.
vs.
if the 1st character is '.' and the 2nd character is '.' and the 3rd
character is not end of line, remove the first character.
[TR] This might be needed to throw an error.
NOTE the input set precondition in the derived inverse function. If
f'(y) is not defined for y=1 then how can it be expected to give a
result for y=1.
The input *is* defined. <<Any line starting with a dot and more than
one character>> is to be modified.
[TR] It is not correct inverse operation and the suggestion is to
correct it.
The output is not defined. The sending MTA should have done something
different, not the receiving one.
[TR] Yes, I agree and that is why I would not call receiving MTA as
buggy in this case.