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2 The Language: Rationale and Fundamentals 57
Definition 9. (Reset net). A reset net is a tuple .P;T;F;R/,where

.P;T;F/ is a classical Petri net with a finite set of places P , a finite set of
transitions T , and a flow relation F .P T/ [ .T P/,and
R 2 T ! 2 P is a function defining the reset arcs associated with each transi-
tion t.
A reachable marking M is defined by first removing the tokens needed to enable t
0
from its input places t, then removing all tokens from reset places associated with
t (i.e., R.t/), and finally by adding tokens to its output places t . The notions of
enablement and firing in a reset net can be formalized as follows.
Definition 10. (Reset net enabling and firing rules).Let N D .P;T;F;R/ be a
reset net and M be a marking of the net.
A transition t 2 T is enabled if and only if t M.
An enabled transition t in state M can fire changing the state to M , denoted
0
t
M ! M , if and only if t M and M D .M t/ŒP n R.t/ C t .
0
0
Definition 11. (Occurrence sequence). Let N D .P;T;F;R/ be a reset net with
marking M 0 .Let M 1;::: ;M n be markings of the reset net and t 0 ;t 1 ;::: ;t n 1 be
transitions in T . Sequence D M 0 t 0 M 1:::t n 1 M n is an occurrence sequence if and
t i
only if M i ! M iC1 for all i; 0 i n 1.Amarking M is reachable from a
0

marking M, written M ! M , if and only if there is an occurrence sequence with
0
initial marking M and final/last marking M .
0
The three formalisms described above provide the formal basis for the YAWL lan-
guage and allow business process models developed using the YAWL language to be
enacted in an unambiguous way. In the next three sections, we discuss the control-
flow, data, and resource perspectives of the YAWL language in detail and relate them
to the range of control-flow, data, and resource patterns introduced earlier.

2.4 Control-flow

The control-flow patterns provide a guide to the range of features that are desirable
in a workflow language. YAWL provides an intuitive range of constructs for describ-
ing the control-flow aspects of a business process and in doing so directly supports
31 control-flow patterns. In this section, we discuss the range of control-flow con-
structs that underpin YAWL and provide both examples of their usage and a precise
operational definition for each of them.

2.4.1 Constructs

Figure 2.20 identifies the complete set of language elements that make up the
control-flow perspective of YAWL. A YAWL model or specification is composed