Interoperable interactive geometry for Europe
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free_line(l) states that line l can be moved freely with the mouse.

Several DGS do not have interfaces for moving lines, but resort to moving points and then draw the line passing by those points. Hence, this constraint is not implemented by many DGS. See also line_through_point for another example of a constraint that most DGS cannot implement because their user interface does not allow for it.

Although the free_point and free_line constraints may seem unnecessary, if no constraint is used the behaviour of the element is unspecified. They are also useful in writing the file and in doing topological sort, so please use them.