\section{Related works}
\label{sec:relatedwork}

%external environmental factors
Previous works focus on the external environmental factors that impact the
compositionality of emerged symbolic language. 
For example, XXX proposed small vocabulary sizes~\cite{}.
XXX proposed memoryless~\cite{}.
XXX proposed carefully constructed distractors~\cite{}.
XXX proposed ease-of-teaching~\cite{}.
Such factors are deliberately designed, which are too ideal to be true in
the real world. None of these works realizes the importance of model capacity of
agent itself. \rmk{this should be largely emphasized.}


%measure
Additionally, to measure the compositionality of emerged symbolic language, many metrics are
proposed~\cite{}. 
Widely accepted metrics can be classified into two categories, measuring
positive signaling~\cite{} and measuring positive listening~\cite{}. The former
metrics measure the relationship between spoken symbols and received concepts
\rmk{not clear}, from the perspective of \emph{speakers}. 
For example,.
The latter metrics measure the relationship between received symbols and
predicted concepts \rmk{not clear}, from the perspective of \emph{listeners}.
For example,. 
However, these metrics are not appropriate, for they only measure
compositionality of symbolic language in \emph{unilateral} role\rmk{not sure},
either speakers or listeners. They can not measure the degree of \emph{bilateral}
understanding between speakers and listeners, i.e., the concept-symbol mapping
consistency between speakers and listeners.



