code: public boolean isReadOnly(ELContext context,
                                Object base,
                                Object property) {
          if (context == null) {
              throw new NullPointerException();
          }
  
          if ((base == null) && ("pageContext".equals(property) || 
                  "pageScope".equals(property)) ||
                  "requestScope".equals(property) ||
                  "sessionScope".equals(property) ||
                  "applicationScope".equals (property) ||
                  "param".equals (property) ||
                  "paramValues".equals (property) ||
                  "header".equals (property) ||
                  "headerValues".equals (property) ||
                  "initParam".equals (property) ||
                  "cookie".equals (property)) {
              context.setPropertyResolved(true);
              return true;
          }
          return false;	// Doesn't matter
      }
docs: If the base object is <code>null</code>, and the property matches
  the name of a JSP implicit object, returns <code>true</code>
  to indicate that implicit objects cannot be overwritten.
  
  <p>The <code>propertyResolved</code> property of the
  <code>ELContext</code> object must be set to <code>true</code> by
  this resolver before returning if an implicit object is matched. If
  this property is not <code>true</code> after this method is called,
  the caller should ignore the return value.</p>
  
  @param context The context of this evaluation.
  @param base Only <code>null</code> is handled by this resolver.
  Other values will result in an immediate return.
  @param property The name of the implicit object.
  @return If the <code>propertyResolved</code> property of
  <code>ELContext</code> was set to <code>true</code>, then
  <code>true</code>; otherwise undefined.
  @throws NullPointerException if context is <code>null</code>.
  @throws ELException if an exception was thrown while performing
  the property or variable resolution. The thrown exception
  must be included as the cause property of this exception, if
  available.