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  <xs:element id="ck0000908406_C000144167Member" abstract="true" name="C000144167Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0000908406_BloombergUS13YearGovernmentCreditBondIndexReflectsNoDeductionForFeesExpensesOrTaxesIndexMember" abstract="true" name="BloombergUS13YearGovernmentCreditBondIndexReflectsNoDeductionForFeesExpensesOrTaxesIndexMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0000908406_S000006582Member" abstract="true" name="S000006582Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0000908406_C000017975Member" abstract="true" name="C000017975Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0000908406_C000162988Member" abstract="true" name="C000162988Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="ck0000908406_C000162989Member" abstract="true" name="C000162989Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
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</xs:schema>
