bpel extension - help!!!

[devilva]

hi!!
i'm writing an xsd (see the bottom of topic)
and i want to use it into a bpel process...but when i use this tag,
the XML validator tells me:
setup must contain at least one child activity!!
do you know the meaning?

Thanks!!

----> xsd
<xsd:schema xmlns="http://enterprise.netbeans.org/bpel/esempio"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://enterprise.netbeans.org/bpel/esempio"
xmlns:bpel="http://docs.oasis-open.org/wsbpel/2.0/process/executable"
elementFormDefault="qualified">

<xsd:import namespace="http://www.w3.org/XML/1998/namespace"
schemaLocation="http://www.w3.org/2001/xml.xsd" />
<xsd:import
namespace="http://docs.oasis-open.org/wsbpel/2.0/process/executable"
schemaLocation="http://docs.oasis-open.org/wsbpel/2.0/OS/process/executable/ws-bpel_executable.xsd" />


<xsd:element name="setup" type="tSetup" />

<xsd:complexType name="tSetup">
<xsd:sequence>
<xsd:element name="my_element" type="tMyElement" minOccurs="1"/>
</xsd:sequence>
</xsd:complexType>

<xsd:complexType name="tMyElement">
<xsd:attribute name="smt" type="xsd:string" use="required" />
</xsd:complexType>