snippet.reb

REBOL []
blk: [a b c d]
indexes: [1 3]
result: collect [
    foreach index indexes [
        keep blk/:index
    ]
]
probe result

map-each index indexes [ blk/:index ]

map-each x [[1 2 3] [4 6 5] [7 8 9]] [apply :reduce [first maximum-of x]]