[Tutor] Error-handling for a large modular program

[Shrutarshi Basu]

Yes  I am aware of the various Python parser packages. But at the
current moment the language is changing so I'm just writing a parser
as I go along. It probably isn't very good as it combines syntactical
analysis with some calculations as it goes along (which isn't really
good practice). Once the language design is finalized, I might look
into using a proper parser and performing the calculations separately.