# Why doesn't f(alpha) act like a function?

var('t,alpha,beta,gamma')

sage: i [ 0 -I] [-I 0]

sage: f(alpha) = cos(alpha)*i

sage: f(alpha)
[ 0 -I*cos(alpha)]
[-I*cos(alpha) 0]

sage: f(0)
[ 0 -I*cos(alpha)]
[-I*cos(alpha) 0]

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