constraint error question for language lawyers

[blakemor@software.org (Alex Blakemore)]

OK, language lawyers of the world.
This one has stumped everyone I've asked locally,  including several 
people who have worked with Ada since there were compilers for it.

Vax Ada 2.1 and two versions of Verdix (5.x and 6.0.3) all behave in the same
(unreasonable) manner - so there must be some obscure rule at work here.
Does anyone
  a. know what that rule might be ?
  b. profess to have a reasonable explanation for its existence.

The Question:
  Why does this procedure raise constraint error?
  It happens on the second assignment to dummy, but
  doesnt happen if dummy.len > 0 and appears to have 
  something to do with the function call "&"

----------------  cut here  -----------------

with text_io;
use text_io;

procedure bozo is
  
  type short is range 0 .. 10;

  type data is array (short range <>) of character;

  type var_text (len : short := 0) is
    record
      text : data (1 .. len);
    end record;

  dummy : var_text; -- unconstrained

  procedure do_nothing (d : data) is 
  begin
    null;
  end do_nothing;

begin

  put_line ("before");
  dummy := (len => 0, text => "");
  put_line ("during");
  dummy := (len  => dummy.len + 1,
            text => dummy.text & 'a');
  put_line ("after");

exception

  when constraint_error =>
    put_line ("constraint_error raised");

end bozo;

---------------- cut here -----------------

Here is the output:

before
during
constraint_error raised

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Alex Blakemore                       CSNET:   blakemore@software.org
Software Productivity Consortium     
2214 Rock Hill Road Herndon, VA 22070 (703) 742-7125