Grapher: How to revise definitions

Hi there,

I have no problems in writing definitions and equations in grapher. But when I tried to revise definitions, error message shows saying "Cannot remove definition. The definition is used by another element." But I did not remove, just replace a constant number with a new parameter. For example,

The original one:
y=x+b
b:=1

Now I want it to be:
y=x+b
b:=a
a:=2

But when I try to revise the definition of b, grapher does not allow me showing the above error message.

Anyone has the same problem and can help me with that?

Thanks a lot!!

Rita