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for i ( 0 .. n ) {
    f( a[i] );  // the required stuff for a[]
    f'( b[i] );  // the required stuff for b[]
    g( a[i], b[i] ); // and whilst we have them, do something to, or a
+ccumulate some statistic, that might help the subsequent merge.
}

f( a[0] ); f'( b[0] );
for i ( 1 .. n ) {
...
    g( a[i], a[i-1] );
    g'( b[i], b[i-1] );
}