================================================================
ABSENT-HANDLING

mlr put $a = $a * 2
a=2,b=2
a=,b=4
x=,b=6,a=2
a=14,b=8

mlr put $new = $a * 2
a=1,b=2,new=2
a=,b=4,new=
x=,b=6,new=2
a=7,b=8,new=14

mlr put $new = @nonesuch * 2
a=1,b=2,new=2
a=,b=4,new=2
x=,b=6,new=2
a=7,b=8,new=2

mlr put $new = @nonesuch * $a
a=1,b=2,new=1
a=,b=4,new=
x=,b=6,new=
a=7,b=8,new=7

mlr put @sum = @sum + $a; emit @sum
sum=1
a=1,b=2
sum=
a=,b=4
sum=
x=,b=6
sum=
a=7,b=8

mlr put @product = @product * $a; emit @product
product=1
a=1,b=2
product=
a=,b=4
product=
x=,b=6
product=
a=7,b=8

mlr put begin { @sum = 10 }; @sum = @sum + $a; emit @sum
sum=11
a=1,b=2
sum=
a=,b=4
sum=
x=,b=6
sum=
a=7,b=8

mlr put begin { @product = 10 }; @product = @product * $a; emit @product
product=10
a=1,b=2
product=
a=,b=4
product=
x=,b=6
product=
a=7,b=8