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This is an extremely challenging task because -LRB- i -RRB- visual comparisons not only need to be fine-grained but also executed cross-domain , -LRB- ii -RRB- free-hand -LRB- finger -RRB- sketches are highly abstract , making fine-grained matching harder , and most importantly -LRB- iii -RRB- annotated cross-domain sketch-photo datasets required for training are scarce , challenging many state-of-the-art machine learning techniques .
 used for: annotated cross-domain sketch-photo datasets, machine learning techniques
Given a phrase that describes the relationship between two words, extract the words and the lexical relationship between them. The output format should be "relation1: word1, word2; relation2: word3, word4". 
Option: conjunction, feature of, hyponym of, used for, part of, compare, evaluate for 
Text: {0} 
Answer:
 used for: annotated cross-domain sketch-photo datasets, machine learning techniques