A list of an expression is evaluable in gprolog. Is this a bug?

[Dave Sworin]

I cannot find any justification for the behavior below in the
GNU Prolog Manual 1.3.0 where a list of a number or _expression_
is evaluable.  Is this a bug or an undocumented feature?

(See many examples below.)

 

address@hidden ceg]$ gprolog
GNU Prolog 1.3.0
By Daniel Diaz
Copyright (C) 1999-2007 Daniel Diaz
| ?- X is [1.0,2.0,3.0]/2.
uncaught exception: error(type_error(evaluable,'.'/2),(is)/2)
| ?- X is [4.0]/2.
 
X = 2.0
 
(1 ms) yes
| ?- X is [4.0].
 
X = 4.0
 
(1 ms) yes
| ?- X is [1.0 + 2.0 + 3.0].
 
X = 6.0
 
(1 ms) yes
| ?-

 

Dave