Add description of record constructors

This closes #60

Author: mtiller

text/source/behavior/equations/record_def.rst

@@ -47,5 +47,33 @@ The following are all examples of ``record`` definitions:
 Record Constructors
 -------------------
 
-.. todo:: I need to fill this in
+.. index:: ! record constructor
 
+Now that we know how to define a ``record``, we need to know how to
+create one.  If we are declaring a variable that happens to be a
+``record``, the declaration itself will create an instance of the
+``record`` and we can specify the values of variables inside the
+record using modifications, *e.g.*,
+
+.. code-block:: modelica
+
+    parameter Vector v(x=1.0, y=2.0, z=0.0);
+
+But there are some cases where we might want to create an instance of
+a ``record`` that isn't a variable (*e.g.*, to use in an expression,
+pass as an argument to a function or use in a modification).  For each
+``record`` definition, a function is automatically generated with the
+**same name** as the ``record``.  This function is called the "record
+constructor".  The record constructor has input arguments that match
+the variables inside the ``record`` definition and returns an instance
+of that ``record``.  So in the case of the ``Vector`` definition
+above, we could also initialize a ``parameter`` using the record
+constructor as follows:
+
+.. code-block:: modelica
+
+    parameter Vector v = Vector(x=1.0, y=2.0, z=0.0);
+
+In this case, the value for ``v`` comes from the **expression**
+``Vector(x=1.0, y=2.0, z=0.0)`` which is a call to the record
+constructor.