code: static ModelNode resolveExpressions(final ModelNode unresolved, final ExpressionResolver expressionResolver, boolean excludePostBootSystemProps) {
  
          ModelNode toResolve = unresolved.clone();
          ModelNode sysProps = null;
          if (excludePostBootSystemProps && toResolve.hasDefined(SYSTEM_PROPERTY)) {
              sysProps = toResolve.remove(SYSTEM_PROPERTY);
          }
          try {
              ModelNode result = expressionResolver.resolveExpressions(toResolve);
              if (sysProps != null) {
                  ModelNode resolvedSysProps = new ModelNode().setEmptyObject();
                  for (Property property : sysProps.asPropertyList()) {
                      ModelNode val = property.getValue();
                      boolean bootTime = SystemPropertyResourceDefinition.BOOT_TIME.resolveModelAttribute(expressionResolver, val).asBoolean();
                      if (bootTime) {
                          val.get(VALUE).set(SystemPropertyResourceDefinition.VALUE.resolveModelAttribute(expressionResolver, val));
                      }
                      // store the resolved boot-time to save re-resolving later
                      val.get(BOOT_TIME).set(bootTime);
                      resolvedSysProps.get(property.getName()).set(val);
                  }
                  result.get(SYSTEM_PROPERTY).set(resolvedSysProps);
              }
              return result;
          } catch (OperationFailedException e) {
              // Fail
              throw new IllegalStateException(e.getMessage(), e);
          }
      }
docs: Resolve expressions in the given model (if there are any)
  
  @param unresolved node with possibly unresolved expressions. Cannot be {@code null}
  @param expressionResolver resolver to use. Cannot be {@code null}
  @param excludePostBootSystemProps {@code true} if child system-property nodes should be checked
  for the 'boot-time' attribute, with resolution being
  skipped if that is set to 'false'. WFCORE-450
  
  @return a clone of {@code unresolved} with all expression resolved