expr [ number ]
] are ordinary square brackets.
 operator creates a property that takes a structure as input and returns one of the objects of this structure. Objects are accessed using the sequence number of the object.
An expression whose value must be a structure.
The sequence number of an object. Integer literal. Must be within the range of
Nis the number of objects in the structure.
attachment1 = DATA FILE (Letter);
attachment2 = DATA FILE (Letter);
letterAttachments (Letter l) = STRUCT(attachment1(l), attachment2(l));
secondAttachment(Letter l) = letterAttachments(l); // returns attachment2