Fix error.

src/AlgebraicGeometry/Schemes/Methods.jl

@@ -0,0 +1,39 @@
+@doc raw"""
+    base_change(phi::Any, X::Scheme)
+
+For a `Scheme` ``X`` over a `base_ring` ``𝕜`` and a map ``φ : 𝕜 → R`` 
+we compute ``X' = X ×ₖ Spec(R)`` and return a pair `(X', f)` where 
+``f : X' → X`` is the canonical morphism.
+
+!!! note
+We do not restrict `phi` to be a `Hecke.Map` so that one can also use 
+coercion, anonymous functions, etc. 
+"""
+function base_change(phi::Any, X::Scheme)
+  error("base_change not implemented for input of type $(typeof(X))")
+end
+
+@doc raw"""
+    base_change(phi::Any, f::SchemeMor;
+        domain_map::SchemeMor, codomain_map::SchemeMor
+      )
+
+For a morphism ``f : X → Y`` with both ``X`` and ``Y`` defined over a 
+`base_ring` ``𝕜`` and a map ``φ : 𝕜 → R`` return a triple `(a, F, b)` 
+where ``a : X' → X`` is the morphism from `base_change(phi, X)`, 
+``b : Y' → Y`` the one for ``Y``, and ``F : X' → Y'`` the induced 
+morphism on those fiber products.
+
+!!! note
+We do not restrict `phi` to be a `Hecke.Map` so that one can also use 
+coercion, anonymous functions, etc. 
+
+!!! note 
+The morphisms ``a`` and ``b`` can be passed as the optional arguments 
+`domain_map` and `codomain_map`, respectively. 
+"""
+function base_change(phi::Any, f::SchemeMor;
+    domain_map::SchemeMor, codomain_map::SchemeMor
+  )
+  error("base_change not implemented for input of type $(typeof(f))")
+end