pic rejects nested expr<posn,posn>

[M. Douglas McIlroy]

pic 1.20.1 likes centroid1, but reports a syntax
error on expanding centroid2.  Both should be OK 
according to the syntax given in pic.ps that comes
with the distribution.

.PS
define centroid1 { (2/3)<$3,(1/2)<$1,$2>> }
define centroid2 { (1/3)<(1/2)<$1,$2>,$3> }
A: "A" at 0,0
B: "B" at 1,0
C: "C" at 0,1
"+" at centroid1(A,B,C)
"+" at centroid2(A,B,C)
.PE