assignment operator and function definition



I'm still struggling to understand how to use the assignment operator ":" properly.

Why does the following not give y(x) := x^2 + x - 1 ?

Maxima 5.22.1 http://maxima.sourceforge.net
using Lisp GNU Common Lisp (GCL) GCL 2.6.8 (a.k.a. GCL)
Distributed under the GNU Public License. See the file COPYING.
Dedicated to the memory of William Schelter.
The function bug_report() provides bug reporting information.

(%i1) 
kill(all);
ex1: x^2;
ex2: x -1;
y(x):=ex1+ex2;

(%o0)                                done
(%i1) 
                                       2
(%o1)                                 x
(%i2) 
(%o2)                                x - 1
(%i3) 
(%o3)                          y(x) := ex1 + ex2
(%i4)