Fixtype of translated functions.
This is a tagged union type, introduced by fty::deftagsum.
The `translated' adjective refers to the fact that
this is a function in a translated term (see tterm).
A translated function is a named function (any symbol)
or a lambda expression;
the latter consists of zero or more parameters (any symbols)
and a body (any term).
We do not constrain a named function
to differ from the symbol
Note that we use symbol values, not symbols, for the same reason explained in tterm.