SendExpression :
    CSN1String  '='  CSN1String
|   CSN1String  'send'  CSN1String

The '=' and 'send' operators are equivalent. The left hand operand represents the string that should be matched by a decoder. The right hand operand represents the string that should be sent by an encoder. Naturally, the right hand operand should provide a value that can be accepted by the left hand operand.