Sequence diagrams consist of objects, their lifelines (also known as smimming lanes), and the exchanged messages. Sequence diagrams are defined in UMLGraph using the pic syntax.

A sequence diagram file must start with the sequence: .PS copy "sequence.pic"; The `.PS` marks the beginning of pic commands. The sequence `copy "sequence.pic";` loads the macros defining the sequence diagram operations. The file `sequence.pic`, part of the UMLGraph distribution, must exist in the directory where pic2plot will be executed.

Sequence diagram files must end with the sequence: .PE

The sequence diagrams are defined by calling function-like pic 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.

As an example, the following defines an object `O` that will be identified in the diagram as `o:Toolkit` object(O,"o:Toolkit");

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