See the IBM Reference, page 280, paragraph 3.
When ⎕EC executes an expression that signals a user-defined error, the
second and third items of the result should be the same as the
expression's ⎕ET (which would be 0 1 in this case) and ⎕EM. GNU APL
returns 0 0 and 'User defined error'.
Note also that ⎕ET must not be changed by ⎕EC. GNU APL already does this
correctly.
It's not clear from the IBM Reference whether ⎕EM should also not be
changed by ⎕EC. It seems reasonable to assume that *not* changing ⎕EM is
the correct behavior; the whole point of ⎕EC is to execute an expression
in a controlled manner. If so, GNU APL is already correct in this
regard.
⎕es 'foo'
foo
⎕ES 'foo'
^
⎕em
foo
⎕ES 'foo'
^
⎕et
0 1
⎕ec '⎕es ''foo'''
0 0 0 User defined error
⎕em
