i1 : -- map defined by the quadrics through a twisted cubic curve
ZZ/33331[a..d]; f = rationalMap {c^2-b*d,b*c-a*d,b^2-a*c};
o2 : RationalMap (quadratic rational map from PP^3 to PP^2)
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i3 : Phi = rationalMap {f,f};
o3 : MultirationalMap (rational map from PP^3 to PP^2 x PP^2)
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i4 : time isIsomorphism Phi
-- used 0.00040967s (cpu); 8.49e-06s (thread); 0s (gc)
o4 = false
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i5 : Psi = first graph Phi;
o5 : MultirationalMap (birational map from threefold in PP^3 x PP^2 x PP^2 to PP^3)
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i6 : time isIsomorphism Psi
-- used 0.515351s (cpu); 0.301114s (thread); 0s (gc)
o6 = false
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i7 : Eta = first graph Psi;
o7 : MultirationalMap (birational map from threefold in PP^3 x PP^2 x PP^2 x PP^3 to threefold in PP^3 x PP^2 x PP^2)
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i8 : time isIsomorphism Eta
-- used 2.23565s (cpu); 1.34328s (thread); 0s (gc)
o8 = true
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i9 : assert(o8 and (not o6) and (not o4))
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