#' @details
#'
#' @section Measures of triad closure: Each measure of triad closure is defined
#' as the proportion of wedges that are closed, where a \emph{wedge} is the
#' image of a specified two-event triad \eqn{W} under a specified subcategory
#' of graph maps \eqn{C} subject to a specified congruence relation \eqn{~},
#' and where a wedge is \emph{closed} if it is the image of such a map that
#' factors through a canonical inclusion of \eqn{W} to a specified self-dual
#' three-event triad \eqn{X}.
#'
#' The alcove, wedge, maps, and congruence can be specified by numerical codes
#' as follows (no plans exist to implement more measures than these):
#' \itemize{
#' \item{\code{alcove}:
#' \itemize{
#' \item{\code{0}: \eqn{T_{(1,1,1),0}}}
#' \item{\code{1}: \eqn{T_{(1,1,0),1}}} (\strong{not yet implemented})
#' \item{\code{2}: \eqn{T_{(1,0,0),2}}} (\strong{not yet implemented})
#' \item{\code{3}: \eqn{T_{(0,0,0),3}}} (\strong{not yet implemented})
#' }
#' }
#' \item{\code{wedge}:
#' \itemize{
#' \item{\code{0}: \eqn{T_{(1,1,0),0}}}
#' \item{\code{1}: \eqn{T_{(1,0,0),1}}} (\strong{not yet implemented})
#' \item{\code{2}: \eqn{T_{(0,0,0),2}}} (\strong{not yet implemented})
#' }
#' }
#' \item{\code{maps}:
#' \itemize{
#' \item{\code{0}: all graph maps (injective on actors)}
#' \item{\code{1}: injective graph maps}
#' \item{\code{2}: induced injective graph maps}
#' }
#' }
#' \item{\code{congruence}:
#' \itemize{
#' \item{\code{0}: same actor and event images (equivalence)}
#' \item{\code{1}: same actor images, structurally equivalent event images}
#' \item{\code{2}: same actor images}
#' }
#' }
#' }
#' Some specifications correspond to statistics of especial interest:
#' \itemize{
#' \item{\code{0,0,0,2}:
#' the classical clustering coefficient (Watts & Strogatz, 1998),
#' evaluated on the unipartite actor projection
#' }
#' \item{\code{0,0,1,0}:
#' the two-mode clustering coefficient (Opsahl, 2013)
#' }
#' \item{\code{0,0,2,0}:
#' the unconnected clustering coefficient (Liebig & Rao, 2014)
#' }
#' \item{\code{3,2,2,0}:
#' the completely connected clustering coefficient (Liebig & Rao, 2014)
#' (\strong{not yet implemented})
#' }
#' \item{\code{0,0,2,1}:
#' the exclusive clustering coefficient (Brunson, 2015)
#' }
#' \item{\code{0,0,2,2}:
#' the exclusive clustering coefficient
#' }
#' }
#' See Brunson (2015) for a general definition and the aforecited references for
#' discussions of each statistic.
#'
#' @references
#'
#' Watts, D.J., & Strogatz, S.H. (1998). Collective dynamics of "small-world"
#' networks. \emph{Nature}, 393(6684), 440--442.
#'
#' Opsahl, T. (2013). Triadic closure in two-mode networks: Redefining the
#' global and local clustering coefficients. \emph{Social Networks}, 35(2),
#' 159--167. Special Issue on Advances in Two-mode Social Networks.
#'
#' Liebig, J., & Rao, A. (2014). Identifying influential nodes in bipartite
#' networks using the clustering coefficient. Pages 323--330 of:
#' \emph{Proceedings of the tenth international conference on signal-image
#' technology and internet-based systems}.
#'
#' Brunson, J.C. (2015). Triadic analysis of affiliation networks. \emph{Network
#' Science}, 3(4), 480--508.
#'
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