Relational operators
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% 1. Arithmetic operator
% 2. Relational operators (==, ~=, >, >=, <, <=) from left to right
% 3. all ~ operators
% 4. & and &&? from left to right
% 5. | , || and xor from left to right
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% NB As for arithmetic operators, parentheses can be used to change the
% default order.