 Usage:  a = andrews(x,y,id,b,h)  
 
 Input:

  x                      n x M regressor matrix. WARNING: x may not contain a vector of ones! 
                         
  id                     n x 1 vector containing the estimated index of the first-step selection equation. 
                         
  y                      n x 1 matrix containing n observ. of the dependent variable 
                         
  b                      M x 1 vector of estimated slope coefficients 
                         
  h                      scalar, the bandwidth 
                         
 Output:

  a                      scalar, estimated intercept coefficient 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
