oiskuu if I run your code on this data:- 
__DATA__
M7,    Q,    P,
M7,    M28,    E,
M28,    M6,    E,
M6,    Q,    Pl,

[download]
I get this 
M7<-M28<-M6<-Q = Pl.E.E               
M6<-Q = Pl                            
M28<-M6<-Q = Pl.E

[download]
Whereas I expect 
M7<-Q = P
M28<-M6<-Q = Pl.E
M6<-Q = Pl

[download]
That is I expect only the shortest path from child to parent if it is there.

In reply to Re^2: constructing a tree from csv file by zing
in thread constructing a tree from csv file by zing

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