%$%                     Exposition pipe
%<>%                    Assignment pipe
%>%                     Pipe
build_df_cooc           Compute monthly co-occurrence matrix
build_spm_cooc_sym      Build symmetric sparse matrix from data frame
get_pmi                 Compute pointwise mutual information (PMI)
get_svd                 Compute random singular value decomposition
                        (rSVD)
spm_to_df               Write sparse matrix to dataframe
sql_cooc                Compute co-occurrence matrix on SQL file
