I have seen that lambdify wants a list for the arguments to be treated as
symbols. This list can be also give as a python set, e.g. {x,y,z }, which
is exactly what would be returned by a .free_symbol property. If lambdify
is feed a set like that of the output of .free_symbol it may change their
order in the conversion from set to list. This is done silently and may
cause major disfunction in the use of the lamdified function because you
think x is x, but is y instead and so on.
I would like to ask developers to check for the type of the lambdify list
of arguments and throw a warning to flag that a set is being converted and
that the ordering is not guaranteed.
Given that .free_symbols returns a set, not a list, this is very common
pitfall in my opinion and must be prevented.
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