fluid-let[syntax] fluid-let ((var val) ...) exp1 exp2 ...
ChezScheme: simulates dynamically scoped variables.
fluid-let doesn't create new bindings;
var ... should have defined in the lexical scope where fluid-let
appears.
Chicken, MzScheme, Gauche, STk and STklos also has this.
Guile has make-fluid, fluid-ref, fluid-set! and with-fluids*
to do similar job.
Some implementations have a mechanism called parameter, which can
also be used to realize (sort of) dynamic scope. See
make-parameter, parameterize.
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