Help understanding Metagraphs.filter_vertices(g, fn)

Usage First steps
#1

Iā€™m having trouble grasping basic usage of filtering based on v->f(v), MWE:

g = MetaDiGraph(3)
set_props!(g,1,Dict(:a=>"a",:b=>1))
set_props!(g,2,Dict(:šŸ°=>1))
set_props!(g,3,Dict(:šŸ°=>1,:a=>"a"))

#sanity check
@test collect(filter_vertices(g,:šŸ°,1)) ==
	[2,3]
passed

# fn filters
@test collect(filter_vertices(g,v->haskey(props(g,v),:šŸ°) ) ) ==
	[2,3]
error

@test collect(filter_vertices(g,v->haskey(props(g,v),:šŸ°)&&haskey(props(g,v),:a) ) ) ==
	[3]
error