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"pi", "e", "k", ...  are short identifiers. The user may want to use these names for node 
names/labels or in expressions (K for gain or coupling factor, Pi for input power, e for
error...). This can be hard to find, especially in deeply nested subcircuits or when forgetting
that there is no case-sensitivity, or when brackets are optional.

You can use anything you like for net names containing any printable Latin character, except these: (? )? {? } ? '? *? = ,? :? ;? and space. My curiosity showed that even many non-Latin characters and "graphics" like ? and ? are accepted, although these are often not printed properly in the waveform window, which is more limited. You never knew your schematics could be so interesting!

If you try to redefine pi (perhaps being unaware it is already defined), it will succeed. Personally, I think that should generate at least a warning. OTOH, "temp" is also predefined and can be overridden legitimately.