 Usage:  {beta1,beta0,varbeta1,varbeta0} = eivlinearinstr(x,w,y)  
 
 Input:

  x                      n x 1 matrix, the design variables 
                         
  w                      n x 1 matrix, the instrumental variables 
                         
  y                      n x 1 matrix, the response 
                         
 Output:

  beta1                  scalar, the estimate 
                         
  beta0                  scalar, the estimate 
                         
  varbeta1               scalar, the estimate of the variance of the estimate of beta1 
                         
  varbeta0               scalar, the estimate of the variance of the estimate of beta0 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
