Maxima: lambdas and sublist_indices behaving strangely when acting on symbols

Hello,
I'm having trouble using sublist_indices on lists of symbols. ?Basically, I'm trying to implement a function that will find the indices at which a given symbol appears within a list of symbols. ?In other words, I want a function f(l,x) which behaves as follows:
? f([x,y,z], x) -> [1]? f([x,y,z], y) -> [2]? f([x,y,z], z) -> [3]? f([x,y,z], a) -> []
Can anyone explain why the following code should yield [1] instead of []?
? f: lambda([l,x], sublist_indices(l, lambda([z], z = x)))$? f([x,y,z], a); ?/* this yields [1] */
The same function behaves as expected when the arguments are strings:
? f(["x","y","z"], "a"); ?/* this yields [], as expected */
An alternate approach reveals some strange behavior:
? k(l,x) := block(? ? [lstrs: map(string, l), xstr: string(x)],? ? display(l,x,lstrs,xstr),? ? sublist_indices(lstrs, lambda([z], z = xstr))? )$? k([x,y,z], a);
Here, l is displayed as "[x,y,z]", but lstrs is displayed as "[a,y,z]". ?I don't understand why "x" gets changed to "a" here, or how to prevent that substitution. ?(True, I could get around this by just changing the parameter "x" to some other unlikely name, like "uncommonDummyVar", but that seems fragile & inelegant.)
Does anyone have any suggestions on how to implement the behavior I'm looking for?

Thanks,
~ Andrew