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based on an earlier version of the Wolfram Language.
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UpSetDelayed


assigns rhs to be the delayed value of lhs, and associates the assignment with symbols that occur at level one in lhs.
An expression with a delayed definition is evaluated every time it is used:
Make definitions for special and general cases using immediate and delayed assignments:
The definition can be associated with a symbol appearing as an argument of the left side:
The definition can be associated with the head of an argument of the left side:
If several symbols appear in the left-hand side, upvalues for all of them are defined:
^:= defines upvalues:
defines upvalues in the same way as using a tag does:
A tag defines only one upvalue; makes definitions for all symbols:
The right side of an immediate definition is evaluated when the definition is made:
The right side of a delayed definition is evaluated each time the definition is used:
Definitions with the same left side overwrite earlier ones:
Definition prints definitions associated with a symbol:
Information prints various information about a symbol, including any definitions:
UpValues returns a list of rules corresponding to any upvalues defined:
Delayed assignment introduces a scope that is not affected by global variables:
Immediate assignment does not introduce a scope:
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