public Integer getStart() {
        return this.start;
    }
Reference implementation for substring step, a mid-traversal step which returns a substring of the incoming string
traverser with a 0-based start index (inclusive) and optionally an end index (exclusive). If the start index is negative then it will
begin at the specified index counted from the end of the string, or 0 if exceeding the string length. Likewise, if
the end index is negative then it will end at the specified index counted from the end of the string, or 0 if exceeding the string length.
End index is optional, if it is not specified or if it exceeds the length of the string then all remaining characters will
be returned. End index <= start index will return the empty string. Null values are not processed and remain as null when returned.
If the incoming traverser is a non-String value then an {@code IllegalArgumentException} will be thrown.

@author David Bechberger (http://bechberger.com)
@author Yang Xia (http://github.com/xiazcy)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/SubstringLocalStep.java