Allow NULL equivalent entry for examples. When using these to enter new fact expressions, we can specify grammar structure without providing actual populations. This should then ignore the NULL to be stored as a text value and remove the population row. Perhaps user friendly notations such as "***" could be used as a NULL equivalent.
'...' is interpreted as a missing value, this allows the modeler to enter an expression using Qua/Cla, without providing concrete examples. These must be amended in the population editor later on in the modeling process.
