def ambig_map_sample(lang_pair, k=0, r=1.0, pos=set()):
"""
take sample from orignal ambiguity map,
either an absulute number or a fraction,
possibly restricted to particular POS tags
"""
ambig_fname = config["sample"][lang_pair]["ambig_fname"]
ambig_map = AmbiguityMap(ambig_fname)
if pos:
select = [ sl for sl in ambig_map.source_iter()
if sl.rsplit("/",1)[1] in pos ]
else:
select = list(ambig_map.source_iter())
if r < 1.0:
k = int(round(len(select) * r))
elif k == 0:
k = len(select)
select = random.sample(select, k)
ambig_map.source_target_map = dict( (sl, ambig_map[sl])
for sl in select )
return ambig_map
