SimpleTreks -- optional input to choose the type of parametrization in @TO gaussianParametrization@

Put SimpleTreks => true as an argument in the function gaussianParametrization to compute a parametrization of the covariance matrix S=(s_(i,j)) where s_(i,j) is the sum of monomials corresponding to simple treks between vertices i and j. Here, a simple trek is a trek (P_L,P_R) where the paths P_L and P_R do not have any common vertices except perhaps at their source. See trekSeparation for the definition of a trek.