The real-time substitution mechanism begins from the left of the string and finds the first substitution instructions. It evaluates that substitution and comes up with a new version of the string and starts again to evaluate from the left. Nesting of substitution strings may become complex and care should be taken to confirm desired results through testing. An example of using substitution values would be configuring the severity of an ECI to be $V2$, that is, the severity is set to the value of the second keyword/value pair. The severity of an ECI may also use something like $extract(extract(V2,"[",3),"= ",1)$, but this requires a full understanding of the input string and substitution use. A description of the scripting system used within the substitution feature and a complete list of substitution values are listed in Appendix A, “Substitution Scripting and Values.”