I was studying some functions for list manipulation and I have some doubts about the difference between indice, depth and level in an expression. For example, in the detail of DeleteCases there is this:
A positive level n consists of all parts of expr specified by n indices. A negative level -n consists of all parts of expr with depth n.
Can someone explain me what is the difference between these three concepts, an in particular what is the meaning of those details of DeleteCases function.