Circular variable references cause StackOverflowError

[vatbub]

Expected output:
IllegalArgumentException: Circular reference detected

Console output:
Please enter your math expression (Hit Ctrl+C or type 'exit' to quit the program) [default: exit]:
r = r + 1
Exception in thread "main" java.lang.StackOverflowError
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:275)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:288)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:288)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:288)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:288)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)
at com.github.vatbub.mathevaluator.MathExpression.evaluate(MathExpression.java:288)
at com.github.vatbub.mathevaluator.RuntimeConstant.getValue(RuntimeConstant.java:37)