Table 1 The 13 ways of reducing a 4-letter state space ($\mathcal{S}$) to a 3- or 2-letter state space ${\mathcal{S}}^{\prime }$.

{{A, G}, C, T}

R

{R, C, T}

{2 : 1 : 1}

{A, G, {C, T}}

Y

{A, G, Y}

{A, {C, G}, T}

S

{A, S, T}

{C, G, {A, T}}

W

{C, G, W}

{A, C, {G, T}}

M

{M, G, T}

{C, {A, G, T}}

K

{A, C, K}

{A, {C, G, T}}

B

{A, B}

{3 : 1}

{C, {A, G, T}}

D

{C, D}

{G, {A, C, T}}

H

{G, H}

{T, {A, C, G}}

V

{T, V}

{{A, G}, {C, T}}

RY

{R, Y}

{2 : 2}

{{A, T}, {C, G}}

SW

{S, W}

{{A, C}, {G, T}}

KM

{K, M}