RFC-2119
Key words for use in RFCs to Indicate Requirement Levels, S. Bradner, IETF RFC 2119, March 1997
http://www.ietf.org/rfc/rfc2119.txt
BPEL4WS
(BPEL4WS) 1.1, IBM/Microsoft/BEA/SAP/Siebel, May, 2003
http://www-106.ibm.com/developerworks/webservices/library/ws-bpel/