How to do symbolic computation with quaternions

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I need to do symbolic computations with quaternions. Ideally I'd like to enter something like

Q.<i,j,k> = QuaternionAlgebra(QQ,-1,-1)
a=var('a')
b=var('b')
c=var('c')
q = a*i+b*j+c*k

But it doesn't work : TypeError: unsupported operand parent(s) for '*': 'Symbolic Ring' and 'Quaternion Algebra (-1, -1) with base ring Rational Field'

Looks like SAGE doesn't know how to multiply a variable and a quaternion. Or should I specify the types of the variables ? How ?

Thanks for your help.