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<notes>
Sequence diagrams consist of objects, their lifelines
(also known as smimming lanes),
and the exchanged messages.
Sequence diagrams are defined in UMLGraph using the <em>pic</em>
syntax.
<p />
A sequence diagram file must start with the sequence:
<fmtcode ext="pic">
.PS

copy "sequence.pic";
</fmtcode>
The <code>.PS</code> marks the beginning of <em>pic</em> commands.
The sequence
<code>copy "sequence.pic";</code>
loads the macros defining the sequence diagram operations.
The file <code>sequence.pic</code>, part of the UMLGraph distribution,
must exist in the directory where <em>pic2plot</em> will be executed.
<p />
Sequence diagram files must end with the sequence:
<fmtcode ext="pic">
.PE
</fmtcode>
<p />
The sequence diagrams are defined by calling function-like <em>pic</em>
macros.
Each function call is terminated with a semicolon.
Space is not significant between macro calls; on the other hand
adding a space character between a macro's arguments can lead
to surprises and should be avoided.
Objects are referenced using variable-like alphanumeric identifiers.
Strings are enclosed in double quotes.
<p />
As an example, the following defines an object <code>O</code>
that will be identified in the diagram as
<code>o:Toolkit</code>
<fmtcode ext="pic">
object(O,"o:Toolkit");
</fmtcode>
<p />
Comments start with a <code>#</code> character.
<fmtcode ext="pic">
# This is a comment
</fmtcode>
</notes>
