IV-like weighting function for TSLS specification.
sTslsSplines(z, d, j, exz, pi)
vector, the value of the instrument.
0 or 1, indicating treatment or control (redundant in this function; included to exploit apply()).
scalar, position of the component one is interested in constructing the IV-like weight for.
matrix, corresponds to E[XZ'].
matrix, corresponds to E[XZ']E[ZZ']^-1, the first stage regression.
scalar.