auto_stage1 — search of a statistically admissible regression
[listeq]=auto_stage1(y,x,listeq,indx,indcte,firstelim,r00,rmod,alpha,eta,z)
y = vector of the endogenous variable
x = matrix of the exogenous variables
listeq = the list containing all preceding paths and the corresponding estimation results
index = the indexes of the x variables remaining at the entry of the function
indcte = the booelan indicating the presence or the absence of the constant in the regression
firstelim = the index of the first variable which is withdrawn to begin the path
r00 = the results tlist for the regression containing all exogenous variables whose index is in indx
rmod = a tlist defined by def_results
alpha = broad significance level (typically lower than f0_sig)
eta = (px1) significance level for specification tests
z = matrix of the compulsory variables (the ones that must be in the regression whatever significance they have)
listeq = the entry list plus the path followed here and, if it leads to a new model, the corresponding estimation results