code: public Binding lookup(String varName) {
          if (varName.equals(binding.getVarName())) {
              return binding;
          }
          return parent != null ? parent.lookup(varName) : null;
      }
docs: Look for a Binding for given variable.
  
  @param varName
  name of the variable
  @return the Binding, or null if no such Binding is present in the set