Opposite of CONCAT()?

[DrD]

CONCAT(matrix,identity(n)) is used to form an intermediate result. A later operation needs to remove identity(n) from the result:

Example:

a:=[[1,3],[2,7]];
a:=CONCAT(a,identity(2));
a1:=RREF(a); // The concatenation of identity(2), and a^-1

I was hoping for something similar to these ideas, (to remove identity(n) from a concatenation):
// SUPPRESS(a1,identity(2));
// remove(identity(2),a1);
// SUB(a1,a1-identity(2));

Treating identity(n) as an object, fails.