Person( age > ($age * 2) ) ) ### typo - note the redundant ')'

This produces nested Empty Stack and Class Cast Exceptions, lots of stack dumps and three error messages:

[0,0]: Line 30:0 unexpected exception at input 'rule'. Exception: java.util.EmptyStackException. Stack trace:
java.util.EmptyStackException
at java.util.Stack.peek(Stack.java:85)
at java.util.Stack.pop(Stack.java:67)
at org.drools.lang.ParserHelper.popParaphrases(ParserHelper.java:409)
at org.drools.lang.ParserHelper.end(ParserHelper.java:723)
at org.drools.lang.DRLParser.rule(DRLParser.java:915)
at org.drools.lang.DRLParser.statement(DRLParser.java:248)
at org.drools.lang.DRLParser.compilationUnit(DRLParser.java:143)
at org.drools.compiler.DrlParser.compile(DrlParser.java:225)
at org.drools.compiler.DrlParser.parse(DrlParser.java:136)
at org.drools.compiler.DrlParser.parse(DrlParser.java:141)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:307)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:486)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28)
at express.Main.setup(Main.java:48)
at express.Main.main(Main.java:92)

[0,0]: Line 31:8 unexpected exception at input '"mvel"'. Exception: java.lang.ClassCastException: org.drools.lang.api.CEDescrBuilderImpl cannot be cast to org.drools.lang.api.AttributeSupportBuilder. Stack trace:
java.lang.ClassCastException: org.drools.lang.api.CEDescrBuilderImpl cannot be cast to org.drools.lang.api.AttributeSupportBuilder
at org.drools.lang.ParserHelper.start(ParserHelper.java:667)
at org.drools.lang.DRLParser.stringAttribute(DRLParser.java:1294)
at org.drools.lang.DRLParser.attribute(DRLParser.java:1073)
at org.drools.lang.DRLParser.statement(DRLParser.java:257)
at org.drools.lang.DRLParser.compilationUnit(DRLParser.java:143)
at org.drools.compiler.DrlParser.compile(DrlParser.java:225)
at org.drools.compiler.DrlParser.parse(DrlParser.java:136)
at org.drools.compiler.DrlParser.parse(DrlParser.java:141)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:307)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:486)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28)
at express.Main.setup(Main.java:48)
at express.Main.main(Main.java:92)

[31,8]: [ERR 107] Line 31:8 mismatched input '"mvel"' expecting one of the following tokens: '[package, import, global, declare, function, rule, query]'.
[0,0]: Line 39:0 unexpected exception at input 'rule'. Exception: java.util.EmptyStackException. Stack trace:
java.util.EmptyStackException
at java.util.Stack.peek(Stack.java:85)
at java.util.Stack.pop(Stack.java:67)
at org.drools.lang.ParserHelper.popParaphrases(ParserHelper.java:409)
at org.drools.lang.ParserHelper.end(ParserHelper.java:723)
at org.drools.lang.DRLParser.rule(DRLParser.java:915)
at org.drools.lang.DRLParser.statement(DRLParser.java:248)
at org.drools.lang.DRLParser.compilationUnit(DRLParser.java:143)
at org.drools.compiler.DrlParser.compile(DrlParser.java:225)
at org.drools.compiler.DrlParser.parse(DrlParser.java:136)
at org.drools.compiler.DrlParser.parse(DrlParser.java:141)
at org.drools.compiler.PackageBuilder.addPackageFromDrl(PackageBuilder.java:307)
at org.drools.compiler.PackageBuilder.addKnowledgeResource(PackageBuilder.java:486)
at org.drools.builder.impl.KnowledgeBuilderImpl.add(KnowledgeBuilderImpl.java:28)
at express.Main.setup(Main.java:48)
at express.Main.main(Main.java:92)