public int getVariableIdx(final String name, final boolean create) {
        Integer num = variableNumbers.get(name);
        if (num == null) {
            if (create) {
                num = variableNumbers.size();
                variableNumbers.put(name, num);
                lastAssignedExpression.add(null);
            } else {
                return -1;
            }
        }
        return num;
    }
add a new lookup-value for the given name to the given lookupData array.
If no array is given (null value passed), the value is added to
the context-binded array. In that case, unknown names and values are
created dynamically.

@return a newly created value element, if any, to optionally add aliases
main/src/main/java/cgeo/geocaching/brouter/expressions/BExpressionContext.java