Tool for Entity Relationship Diagrams (TERD).
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This tool is intended for entity relationship diagrams (ERDs) that are
used in classic Entity Relationship (ER) modeling.
 
More about the drawing of ERDs can be found in chapter 4 of the
user's guide and more about the use of this notation technique can be 
found in appendix A of the user's guide.


Diagram editing in general.
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Diagrams that are made by a diagram editor are a special
kind of graph consisting of nodes and edges and with a certain
representation. In the representation of a graph, nodes and edges are
shown as shapes (boxes, lines, diamonds etc.).
 
Documents should satisfy certain constraints. Most constraints
are specific for the particular diagram technique supported by the
editor. In the editors there are three kinds of constraints:
 
1. Built-in constraints which are constraints which can never be
violated because there is no command in the user interface to achieve
that.
 
2. Immediately enforced constraints: when you perform a command that
would violate a constraint that is immediately enforced, this command
is rejected immediately by the editor and a pop-up window with an error
message is displayed.
 
3. Soft constraints which are constraints that can be violated.
Soft constraints are checked by the editor when the Check Document
from the Document menu is issued by you. Check Document displays a list
of error messages in a pop-up window. As opposed to the previous two
classes of constraints, you are responsible for correcting the diagram.

