protected static final Set<String> getVariables(final String equation) {
        final Matcher matcher = VARIABLE_PATTERN.matcher(equation);
        final Set<String> variables = new LinkedHashSet<>();
        while (matcher.find()) {
            variables.add(matcher.group());
        }
        return variables;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MathStep.java