Add project files

examples/combined/Expr.g4

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+// A small expression grammar in one combined file.
+//   antlrlab examples/combined/Expr.g4 examples/combined/input.txt
+grammar Expr;
+
+prog : expr EOF ;
+
+expr
+    : expr op=(MUL | DIV) expr   # MulDiv
+    | expr op=(ADD | SUB) expr   # AddSub
+    | INT                        # Int
+    | ID                         # Id
+    | '(' expr ')'               # Parens
+    ;
+
+MUL : '*' ;
+DIV : '/' ;
+ADD : '+' ;
+SUB : '-' ;
+INT : [0-9]+ ;
+ID  : [a-zA-Z_] [a-zA-Z_0-9]* ;
+WS  : [ \t\r\n]+ -> skip ;

examples/combined/input.txt

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+1 + 2 * (width - 3)

examples/combined/input_bad.txt

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+1 + * 2 (width