Find the difference of two lists, i.e. the list of elements that occur in list ''a'' but not in list ''b'' (and ''not'' the intersection, i.e. the list of elements that occur in only one of the two lists).

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proc ldiff {a b} {
    lmap elem $a {
        expr {[lsearch -exact $b $elem] > -1 ? [continue] : $elem}
    }
}

ldiff {2 3 4 5 6 7 8 9 10} {2 3 5 7}
# -> 4 6 8 9 10
ldiff {1 2 3} {2 3 4}
# -> 1
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This command probably isn't very efficient.

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