ok   median of ten known values
ok   min and max are the ends
ok   sd is the SAMPLE sd (n-1)
ok   a float 0.0 is DATA, not absence
ok   the string n/a is absence
ok   a missing field is absence
ok   the ladder opens unfiltered
ok   the ladder descends after that
ok   a threshold outside the round is not offered
ok   keep is STRICTLY less than the threshold (4.0 is OUT)
ok   measured() accepts a file with SOME floats
ok   measured() REFUSES a file with no float
ok   sample is reproducible under its seed
ok   sample is WITHOUT replacement
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stealsub selftest green on darwin (14 arms).