TESTS:
sage: e = SFAElementary(QQ)
sage: e([2,1])^2
e[2, 2, 1, 1]
sage: h = SFAHomogeneous(QQ)
sage: h([2,1])^2
h[2, 2, 1, 1]
sage: p = SFAPower(QQ)
sage: p([2,1])^2
p[2, 2, 1, 1]
sage: QQx.<x> = QQ[]
sage: p = SFAPower(QQx) # indirect doctest
sage: (x*p([2]))^2
x^2*p[2, 2]