WCPS - Introduce LET clause

[Bang Pham Huu]

In WCPS, one can define variables in LET clause which are substitutions in coverage expressions in RETURN clause.

The syntax is:

LET $variable1 := coverageExpression,
    $variable2 := coverageExpression,
    ...

For example:

for $c in (test_mr) 
let $a := 5, 
    $b := 10 
return $a + $b

A special case for subsets when $variable can have this syntax:

LET $variable1 := [domainIntervalList]

And WCPS shorthand subset can apply on this $variable directly, for example:

for $c in (test_mr) 
let $a := [i(20), j(40)], 
    $b := 10 
return encode($c[$a] + $b, "json")