#NEXUS

BEGIN TAXA;
      dimensions ntax=8;
      taxlabels A B C D E F G H;  
END;

BEGIN CHARACTERS;
      dimensions nchar=6;
      format datatype=protein missing=? gap=-;
      charlabels one two three four five six;
      matrix
A     WITH-B
B     WITH-A
C     WITH-D
D     WITH-C
E     WITH-F
F     WITH-E
G     WITH-H
H     WITH-G;
END;

BEGIN TREES;
       tree bush_branchlength_negative = (((A:1,B:1):1,(C:1,D:1):-0.25):1,((E:1,F:1):1,(G:1,H:1):1):1);
END;