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Yup! That's because 2(a-1) does not make sense in Sage. If you had defined f(x)=2+x or something, f(a-1) would be legal.

However, since you were probably trying to do implicit multiplication (as opposed to explicit 2*(a-1)), there is another option. You can turn on implicit multiplication if you wish. But beware - it is fragile! Good luck.