Qualified associations are many-ends of associations that can be reduced to 1 result of the many by sending in qualifier in the brackets at the end of the many-end:

SomeObject.SomeManyObjectsAssociation[someQualifier1,someQualifier2] : OneObject of type element type of the ManyAssociation.

The Qualification is done by locating a method following the pattern Qualify_<SomeManyObjectsAssociation>(someQualifier1,someQualifier2):ObjectType

If no such method is found a generic operator accepting integer as qualifier is used - and the result is corresponding to the at0 operator