Define isaffine methods for AbstractSystem following the pattern

of islinear.

Author: andreasnoack

docs/src/systems/NonlinearSystem.md

@@ -23,6 +23,7 @@ tearing
 ## Analyses
 
 ```@docs
+ModelingToolkit.isaffine
 ModelingToolkit.islinear
 ```
 

docs/src/systems/ODESystem.md

@@ -29,6 +29,7 @@ tearing
 ```@docs
 ModelingToolkit.islinear
 ModelingToolkit.isautonomous
+ModelingToolkit.isaffine
 ```
 
 ## Applicable Calculation and Generation Functions

src/ModelingToolkit.jl

@@ -47,7 +47,7 @@ using Symbolics: _parse_vars, value, @derivatives, get_variables,
                  exprs_occur_in, solve_for, build_expr, unwrap, wrap,
                  VariableSource, getname, variable
 import Symbolics: rename, get_variables!, _solve, hessian_sparsity,
-                  jacobian_sparsity, islinear, _iszero, _isone,
+                  jacobian_sparsity, isaffine, islinear, _iszero, _isone,
                   tosymbol, lower_varname, diff2term, var_from_nested_derivative,
                   BuildTargets, JuliaTarget, StanTarget, CTarget, MATLABTarget,
                   ParallelForm, SerialForm, MultithreadedForm, build_function,

src/systems/abstractsystem.jl

@@ -523,6 +523,12 @@ function islinear(sys::AbstractSystem)
     all(islinear(r, states(sys)) for r in rhs)
 end
 
+function isaffine(sys::AbstractSystem)
+    rhs = [eq.rhs for eq ∈ equations(sys)]
+
+    all(isaffine(r, states(sys)) for r in rhs)
+end
+
 struct AbstractSysToExpr
     sys::AbstractSystem
     states::Vector

test/linearity.jl

@@ -18,3 +18,9 @@ eqs2 = [D(x) ~ σ*(y-x),
        D(z) ~ y - β*z]
 
 @test !ModelingToolkit.islinear(@named sys = ODESystem(eqs2))
+
+eqs3 = [D(x) ~ σ*(y-x),
+        D(y) ~ -z-y,
+        D(z) ~ y - β*z + 1]
+
+@test ModelingToolkit.isaffine(@named sys = ODESystem(eqs))