The AT MOST ONE clause in FEKL is used to create a Fact Type an Internal Uniqueness Constraint but without a Mandatory Role Constraint.
Optionally, if the second Object Type is a Value Type, the Data Type of the Value Type can be defined.

E.g. The following FEKL statement with a AT MOST ONE clause creates the Object-Role Model below:

FEKL: 


Object-Role Model:

NB At this stage the Person Entity Type has no Reference Mode and so is in error and coloured red, and similarly the Name Value Type has no Data Type, and so is in error and is red.

Optionally Defining the Data Type of a Value Type

FEKL:


Object-Role Model:

NB At this stage the Person Entity Type has no Reference Mode and so is in error and coloured red, however the Name Value Type is not in error becausee we have defined the Data Type for the Value Type, and so is not red, but blue.