Add original grammer for part 4

This grammar looks like SDF[0]. Explanation of some constructs:

 - "start-symbol" refers to the start symbol (a non-terminal).
 - Lines below "context-free syntax" refers to the non-terminals.
 - Lines below "lexical syntax" refers to the terminals.
 - "context-free priorities" provides disambiguation constructs.

Note that the "EXP ::= " part really belongs to each individual operand,
so it means something like "A > B > C" (where A, B, C are the unary
minus, multiplication and plus expressions respectively).

I have no idea why Mark van den Brand chose to use multiple
"context-free syntax" and "lexical syntax" groups (if that is
appropriate terminology), as far as I can see all such blocks can be
merged into one group. Maybe he wanted to emphasize the difference
between the left-hand side names?

 [0]: http://releases.strategoxt.org/strategoxt-manual/unstable/manual/chunk-chapter/tutorial-sdf.html

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