  /* compute the cholesky factor of a MA1 correlation matrix
   * Args: 
   *   ma: MA1 autocorrelation 
   *   nrows: number of rows of the covariance matrix 
   * Returns: 
   *   A nrows x nrows MA1 covariance matrix 
   */ 
   matrix cholesky_cor_ma1(real ma, int nrows) { 
     matrix[nrows, nrows] mat; 
     mat = diag_matrix(rep_vector(1 + ma^2, nrows)); 
     if (nrows > 1) { 
       mat[1, 2] = ma; 
       for (i in 2:(nrows - 1)) { 
         mat[i, i - 1] = ma; 
         mat[i, i + 1] = ma; 
       } 
       mat[nrows, nrows - 1] = ma; 
     } 
     return cholesky_decompose(mat); 
   }
