Incomplete lists in Prolog

Hi all,
We are used to being able to create a list with a 'tail' that may 
contain zero or more uninstantiated elements, using syntax like L = [a, 
b, c | Tail]. But why should we not have a list with a section at the 
start or in the middle, maybe even more than one such section, 
containing 0 or more uninstantiated elements? If you go:
L = [a, b, c | Tail], append(L, [d, e, f], L2),
you get a series of solutions with Tail instantiated to different length 
lists. But why shouldn't you get a single solution, something like:
L2 = [a,b,c, '$opening', d,e,f],
which would then unify with any list starting with [a,b,c] and finishing 
with [d,e,f] ? I don't see what is logically different about the end of 
a list from its start or middle, that a representation of an unknown 
number of elements should be allowed there.
    --Jasper