Say I have a list of lists:
{{Subscript[a,1],Subscript[a,1]+Subscript[a,2],Subscript[a,-3]},
{Subscript[a,1],Subscript[a,-2],Subscript[a,-4]}}
Now, in the sublists of that list, if an element has a negative subscript I want to change this element to a negative of that element with a positive subscript, so if I applied this to the list above I'd get:
{{Subscript[a,1],Subscript[a,1]+Subscript[a,2],-Subscript[a,3]},
{Subscript[a,1],-Subscript[a,2],-Subscript[a,4]}}
I don't really know how to do it. Any help would be much appreciated.