termsin_ doesn't work for factors of $-variables

[tueda]

S x,y;
#$F = -x^2+y^2;
#factdollar $F
#$n = termsin_($F);
#$n1 = termsin_($F[1]);
#$n2 = termsin_($F[2]);
#message $F=`$F'
#message $F[0]=`$F[0]'
#message $F[1]=`$F[1]'
#message $F[2]=`$F[2]'
#message $termsin_(F)=`$n'
#message $termsin_(F[1])=`$n1'
#message $termsin_(F[2])=`$n2'
.end

gives

FORM 4.1 (Apr 28 2017, v4.1-20131025-340-gbcde8c9) 64-bits  Run: Thu May 11 20:22:42 2017
~~~$F=y^2-x^2
~~~$F[0]=2
~~~$F[1]=y-x
~~~$F[2]=y+x
~~~$termsin_(F)=2
~~~$termsin_(F[1])=termsin_(y-x)
~~~$termsin_(F[2])=termsin_(y+x)