/ prior extends ': to valence n

prior:{[f]{k#x{@[y;z;:;x[0].|y z+!x 1]}[f]/!k:(-f[1]-1)+#x}}

v:2 4 4 7 9 2 8

g:prior(-;2)
g v
-':v

h:prior({x+y+z};3)
h v