Updating Docs to reference reservered Symbols

[m-messer]

Documentation Request - Peter Johnson

• add the 'hack' that input symbols can be used to allow reserved symbols to be used for different purposes
• reorganise the content so that all info on reserved symbols is together. E.g. overriding Greek symbols; using input symbols; and elementary functions. Maybe there's not a perfect solution, or maybe there's not even a better solution - but it's worth a look.

The 'hack'
If you want to use a symbol that is usually reserved for some elementary function, e.g. ‘E’, do as follows:
Create an input symbol where the code is different that the symbol you want to use, e.g. ‘Ef’ or 'Euler' instead of ‘E’
Add the symbol you want to use as an alternative, e.g. the alternatives could be set to ‘E’

An example:
A/(b*l)

And then add an input symbol:

Symbol: $\epsilon$
Code: b
Alternatives: ϵ,ε,E,e,Ep

All your tests pass with this configuration, e.g. Here the answer A/El is marked as correct.

Note: the 'code' in the input symbol must be a single letter symbol. You can use multi-letter alternatives though.

Now you have a symbol 'b' which you definitely didn't want to show to your students. You can hide it from the input symbols, or replace it with something more palatable. Unfortunately it must be single letter and, unfortunately, it can't be a lower case 'e'. I'm not sure you'll be happy with any particular option.

[m-messer]

See #222 for the proposed changes

[m-messer]

Validated that compare expressions does not currently support custom multiple-character symbols (i.e. bc) with implicit multiplication.

For example, a/bcd with the symbol bc will be processed as b*c*d instead of bc*d. However, if it is a built-in symbol, say theta, it will work, as this is built into SymPy.