This expression cancels the current process instance. Use with care.

    <sequence>
        <participant ref="before" />
        <cancel-process />
        <participant ref="after" />
    </sequence>

the message "after" will never get printed.

Use rather in scenarii like that one :

    class TestDefinition1 < ProcessDefinition
        def make
            process_definition :name => "25_cancel", :revision => "1" do
                sequence do
                    participant "customer"
                    _cancel_process :if => "${f:no_thanks} == true"
                    concurrence do
                        participant "accounting"
                        participant "logistics"
                    end
                end
            end
        end
    end

Note that the expression accepts an "if" attribute.