internal MergeField(FieldStart fieldStart)
{
if (fieldStart.Equals(null))
throw new ArgumentNullException("fieldStart");
if (!fieldStart.FieldType.Equals(FieldType.FieldMergeField))
throw new ArgumentException("Field start type must be FieldMergeField.");
mFieldStart = fieldStart;
// Find the field separator node.
mFieldSeparator = fieldStart.GetField().Separator;
if (mFieldSeparator == null)
throw new InvalidOperationException("Cannot find field separator.");
mFieldEnd = fieldStart.GetField().End;
}