Let D = [0, 1] × [0, 1], and split ∂ D into four line segments γ j. γ γ 1 : [ 0 , 1 ] → → D ; γ γ 1 ( t ) = ( t , 0 ) γ γ 2 : [ 0 , 1 ] → → D ; γ γ 2 ( s ) = ( 1 , s ) γ γ 3 : [ 0 , 1 ] → → D ; γ γ 3 ( t ) = ( 1 − − t , 1 ) γ γ 4 : [ 0 , 1 ] → → D ; γ γ 4 ( s ) = ( 0 , 1 − − s ) so that ∂ ∂ D = γ γ 1 ⊕ ⊕ γ γ 2 ⊕ ⊕ γ γ 3 ⊕ ⊕ γ γ 4