UPDATE:

The command 'y.subs(a)' as used above does not do what I thought. It
just substitutes the variable that comes first in the alphabet with
the definition for a!!!
In this case it did the right thing coincidentally, but not in the
below case:

----------------------------------------------------------------------
| SAGE Version 3.1.4, Release Date: 2008-10-20                       |
| Type notebook() for the GUI, and license() for information.        |
----------------------------------------------------------------------

sage:  var('a b c')
(a, b, c)
sage:  y = a*x^2 + b*x + c
sage:  b = 2*a
sage: y.subs(b)
2*a*x^2 + b*x + c
sage: y.subs({'b': b})
a*x^2 + 2*a*x + c

The .subs() command expects a dictionary such as {'b': b}.

Substitution y(b=b) achieves the same result with much less effort,
but it causes errors in save_session and load_session. Is this a bug?

Cheers
Stan
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