The error function is guaranteed to always terminate the process rather than returning.
Therefore, it should be declared as returning 'a (that is, *anything*) rather than
unit. This means it can be called in e.g. conditional branches where values would otherwise
be required. I see that the last line of the error function is call to Pervasives.exit
(in the manual exit's return type is declared as 'a I believe for this exact reason)
so this is a change to interface only that doesn't require any changes to the implementation.