gistfile1.tcl

namespace path {::tcl::mathop ::tcl::mathfunc}
proc primes n {
    set half [/ [- $n 1] 2]
    set xs [lrepeat $half 1]
    set p 0; set root [/ [sqrt $n] 2]
    while {$p < $root} {
        if {[lindex $xs $p]} {
            set step [+ [* 2 $p] 3]
            set index [+ $p $step]
            while {$index < $half} { lset xs $index 0; incr index $step }
        }
        incr p
    }

    set answer 2
    foreach i [lsearch -all $xs 1] { lappend answer [+ [* 2 $i] 3] }
    set answer
}