C, GAP
extensions = [[['.h', '.hh', '.hpp', '.hxx', '.H'], ['.c', '.cc', '.cpp', '.cxx', '.C']],
              [['.gd'], ['.gi']]];
new = none;

for p in extensions do
  if doc.ext in p[0] then
    new = p[1];
    break;
  elif doc.ext in p[1] then
    new = p[0];
    break;
  fi;
od;

if not new then
  return;
fi;

for e in new do
  file = doc.dir + '/' + doc.base + e;
  if FileExists(file) then
    Open(file);
    return;
  fi;
od;