D = x2fx(X, modelspec)
x2fx creates a design matrix with a constant column and terms requested by the model specification.
X = [1 2; 3 4];
Dlinear = x2fx(X, 'linear')
Dquadratic = x2fx(X, 'quadratic')
X = [1 2; 3 4];
modelspec = [1 0; 0 2; 1 1];
D = x2fx(X, modelspec)