Help with subs

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It's because g(z) = z redefines z as a symbolic expression and g as a callable symbolic expression:

sage: preparse('g(z) = z')
'__tmp__=var("z"); g = symbolic_expression(z).function(z)'

@eric_g: you're right, h(z) = z completely destroys the old meaning of z. I think I might prefer g(z) = ... to raise an error unless I explicitly do var('z') first, but that's not the default behavior.

The problem is similar to the one in this question, and amenable to similar answers.

@john-palmieri: I agree, having h(z) = z silently injecting z in the global namespace is not desirable; I guess that this is a kind of convenience feature that was introduced at the beginning of Sage, but the side effects are too severe IMHO.

@emmanuel-charpentier: it's too bad that subs works with generators of a polynomial ring but not apparently with the named generator of a function field.