A.x != A[0]
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In [1]: from sympy.physics.mechanics import ReferenceFrame
In [2]: A = ReferenceFrame('A')
In [3]: A
Out[3]: A
In [4]: A[0]
Out[4]: A_x
In [5]: type(A[0])
Out[5]: sympy.physics.vector.frame.CoordinateSym
In [6]: A[0] == A.x
Out[6]: False
In [7]: A[0] is A.x
Out[7]: False
In [8]: A.x
Out[8]: A.x
I'm not sure what A[0] is. It must be related to the addition of symbols to support measure numbers that are a function of the spatial location in that reference frame.