overloading - Hiding a field for a subtype in Alloy -

suppose have following signature declarations, in alloy 4.2:

sig target {}  abstract sig {     parent: lone a,     r: target }  sig b extends {} sig c extends {} 

when running, resulting instances have arrows every b target, , c target.

how able hide arrows b?
tried following, @ first:

abstract sig {     parent: lone }  sig b extends {     r: target }  sig c extends {     r: target } 

this give me control on r in b, introduces lot of ambiguity when writing properties. keep these simple possible. example:

all a: | a.r = parent.a.r 

the above says a's targets set of a's children's targets.
latter declarations, have rewrite to

all b: b | b.r = parent.b.((b <: r) + (c <: r)) c: c | c.r = parent.c.((b <: r) + (c <: r)) 

which undesirable.

is there workaround able have general field, still have control on arrows displayed?

you can define function corresponds (c <: r):

fun c_r : -> target {     (c <: r) } 

in alloy visualizer, wil have access relation $c_r. then, can turn off "show arcs" relation r, still have $c_r visible. should hide set of edges belong (b <: r).