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

1 files changed, 30 insertions, 0 deletions

diff --git a/grammar.txt b/grammar.txt
new file mode 100644
index 0000000..0a7a66e
--- /dev/null
+++ b/grammar.txt
@@ -0,0 +1,30 @@
+start-symbol PROGRAM
+
+context-free syntax
+  PROGRAM ::= "begin" DECLS "|" (STATEMENT ";")* "end"
+  DECLS ::= "declare" (ID ",")*
+
+context-free syntax
+  STATEMENT ::= ID ":=" EXP
+
+context-free syntax
+  EXP ::= ID
+  EXP ::= NAT
+  EXP ::= EXP "+" EXP {left}
+  EXP ::= EXP "*" EXP {left}
+  EXP ::= "-" EXP
+  EXP ::= "(" EXP ")"
+
+lexical syntax
+  ID ::= [a-z][a-z0-9]*
+
+lexical syntax
+  NAT ::= [0] | [1-9][0-9]*
+
+lexical syntax
+  LAYOUT ::= [\ \n]*
+
+context-free priorities
+  EXP ::= "-" EXP >
+    EXP ::= "*" EXP >
+    EXP ::= "+" EXP