symbol = s_exp symbol := s_exp symbol ::= s_exp
= is used to assign a value to a variable.
:= is used to assign a value only if no value is currently assigned to the symbol. If the symbol already has a value then the symbol keeps its original value.
::= is used to assign a constant. Once the assignment has been made no changes to the symbol are allowed. Attempted changes to the symbol will generate an error.
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