🚀 go-pugleaf

> Am I right, or have I done something wrong ?

You are right.

The timing at the beginning of a tabular entry is very delicate, TeX
will expand macros (only expand, like in \edef or \write) until it
finds something that isn't expandable. If that is not one of the special
tabular things like \cr (eg from \\) or \crcr (from \end{tabular}) or
\omit (from \multicolumn) then it starts making a new row.

The TeX primitive \input is in fact expandable so that TeX would start
reading the input file without starting a new row, however the LaTeX
\input command does other stuff (such as checking for { } around its
argument) and this means that it is not expandable, So TeX `sees' some
non-expandable token (probably \futurelet if I remember) from the
internal definition of \input and so starts a new row before reading
the input file.

David