feat: add coeff for multivariate laurent polynomials (Nemocas#2218)

Author: thofma

src/generic/LaurentMPoly.jl

@@ -428,6 +428,10 @@ function constant_coefficient(a::LaurentMPolyWrap)
     return coeff(a.mpoly, e)
 end
 
+function coeff(a::LaurentMPolyWrap, e::Vector{Int})
+  return coeff(a.mpoly, e - a.mindegs)
+end
+
 #### exponent vectors
 
 function leading_exponent_vector(a::LaurentMPolyWrap)

test/generic/LaurentMPoly-test.jl

@@ -89,6 +89,9 @@ end
     @test a == sum(coefficients(a) .* monomials(a))
     @test a == L(collect(coefficients(a)), collect(exponent_vectors(a)))
     @test iszero((@inferred constant_coefficient(a)))
+    @test coeff(a, [-2, 1]) == 2
+    @test coeff(a, [1, -3]) == 3
+    @test coeff(a, [1, 1]) == 0
 
     b = MPolyBuildCtx(L)
     for (c, e) in zip(coefficients(a), exponent_vectors(a))