an observation about united modes

[Jose E. Marchesi]

Another advantage of structural type equivalence, and of being able to
express a full mode anywhere a mode specifier is expected, is shown in
the code below, from the same project of mine.

Basically it uses operators and priorities in order to implement a
ternary operator

   x < y IN z

where x and y are symbols and z is an alphabet.  Symbols only hold <
and > relationships with respect some given alphabet, so the trick is to
make

  y IN z

to yield a "qualified symbol", which is a struct (y, z).  Then the
operators < and > are defined to operate on a symbol x and a "qualified
symbol".

Qualified symbols can get created at any time using the operator IN.

{ Relational operators for qualified symbols.

  IN builds a symbol qualified by some given alphabet, which then can
  be used as the second operand of the operators < and >.

  Note that IN shall have a higher priority than the relational
  operators so that ternary expressions such as

    a < b IN e1

  can be written.  }

prio IN = 6;

op IN = (Symbol s, Alphabet a) struct (Symbol s, Alphabet a):
   (s, a);

op < = (Symbol s, struct (Symbol s, Alphabet a) qs) bool:
   (pos_of_symbol_in_word (s, word of a of qs)
    < pos_of_symbol_in_word (s of qs, word of a of qs));

op > = (Symbol s, struct (Symbol s, Alphabet a) qs) bool:
   (pos_of_symbol_in_word (s, word of a of qs)
    > pos_of_symbol_in_word (s of qs, word of a of qs));