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We'll add two relations, one for salt water and one for fresh water.
Top Goals/Undefined in the knowledge browser, in the right hand side and select New Relation (Ctrl+N). In the upper part of the new window, delete any proposal (Quaestor may have filled in the last selected parameter), and enter
Select...
The two relations for the density of the water are only valid for their corresponding watertype. Therefore, a constraint is added to each relation. A constraint is simply a restriction for the validity of the relation.
Top Goals/Undefined in the knowledge browser: all relations in that class are shown (this will work for any class). One can always edit a relation by selecting it and pressing F2 (or and select edit). This is also a useful method to find out which relation you actually selected.
Double click...