Add a human friendly grammar.

Author: hzhangxyz

g4/Ds.g4

@@ -0,0 +1,35 @@
+grammar Ds;
+
+rule_pool
+    : rule* EOF
+    ;
+
+rule
+    : term
+    | term (',' term)* '->' term
+    ;
+
+term
+    : SYMBOL
+    | term '(' (term (',' term)*)? ')'
+    | '(' term ')'
+    | <assoc=right> ('~' | '!' | '-' | '+' | '&' | '*') term
+    | term ('*' | '/' | '%') term
+    | term ('+' | '-') term
+    | term ('<<' | '>>') term
+    | term ('<' | '>' | '<=' | '>=') term
+    | term ('==' | '!=') term
+    | term '&' term
+    | term '^' term
+    | term '|' term
+    | term '&&' term
+    | term '||' term
+    ;
+
+WHITESPACE
+    : [ \t\r\n]+ -> skip
+    ;
+
+SYMBOL
+    :  ~[ \t\r\n,()]+
+    ;