{ t51_strcat: string concatenation, `a + b', in the places its result can go.

  A concatenation needs storage for its result before anything can consume it,
  so each `+' in the source gets its own 256-byte scratch STRING and the result
  IS that variable.  These are the shapes that consume one: as a `writeln'
  argument, assigned to a target, and then used again as an operand.

  The operands cover var+var, literal+var, var+literal, var+char and char+var -
  every pairing a string operand allows except two CHARs, which t53 rejects.
  The CHAR cases are the reason a one-character literal and a CHAR variable
  take the same path: a CHAR operand becomes a one-character string on the
  stack (a length byte and the byte), not a string copied from memory. }
program t51_strcat (input, output);
var
   a, b, c : string[12];
   ch      : char;
begin
   a := 'hello';
   b := 'world';
   ch := 'X';
   writeln (a + b);          { var + var     -> helloworld }
   writeln ('say ' + b);     { literal + var -> say world }
   writeln (a + '!');        { var + literal -> hello! }
   writeln (a + ch);         { var + char    -> helloX }
   writeln (ch + a);         { char + var    -> Xhello }
   c := a + b;               { the result assigned to a variable... }
   writeln (c);
   writeln (b + c)           { ...and then used as an operand }
end.