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Comparison Expressions Comparison expressions return a boolean except for $cmp which returns a number. 

The comparison expressions take two argument expressions and compare both value and type, using the specified BSON comparison order (page 179) for values of different types. 

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Arithmetic Expressions Arithmetic expressions perform mathematic operations on numbers. Some arithmetic ex- pressions can also support date arithmetic. 

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String Expressions String expressions, with the exception of $concat , only have a well-defined behavior for strings of ASCII characters. 

$concat behavior is well-defined regardless of the characters used. 

