What ⩤ means
It keeps pairs whose first elements are outside the set. A relation is a set of ordered pairs. Put the set on the left of ⩤; pairs with a matching first element are removed.
Domain restriction does the reverse: it keeps pairs that start in the chosen set.
⩤ belongs to Z’s specification toolkit
Z grew from Jean-Raymond Abrial’s work in the late 1970s and was developed further at the University of Oxford. It lets software designers describe what a system must do without prescribing its implementation; industrial projects used Z in the 1980s.
A specialist operator used in Z specifications.
In Z, read it as an operator, not ordinary less-than.
Don't confuse it with
4Questions people ask
1Why does ⩤ show as a box on some devices?
Because that device's fonts have no glyph for it. The character is still correct and still copies correctly — the box is a missing glyph, not a broken character.
How to type ⩤
⩤\2A64Codes and encoding1 code points
A visible symbol can contain several code points. JS .length counts UTF-16 units; UTF-8 counts bytes.