Fix #963

Author: YingboMa

src/systems/alias_elimination.jl

@@ -3,13 +3,8 @@ using SymbolicUtils: Rewriters
 const KEEP = typemin(Int)
 
 function alias_elimination(sys)
-    # FIXME: update `structure` too
-    #sys = flatten(sys)
-    #s = get_structure(sys)
-    #if !(s isa SystemStructure)
     sys = initialize_system_structure(sys)
     s = structure(sys)
-    #end
     is_linear_equations, eadj, cadj = find_linear_equations(sys)
 
     v_eliminated, v_types, n_null_vars, degenerate_equations, linear_equations = alias_eliminate_graph(

src/systems/systemstructure.jl

@@ -227,7 +227,7 @@ function find_linear_equations(sys)
             c = expand_derivatives(Differential(var)(term), false)
             # test if `var` is linear in `eq`.
             if !(c isa Symbolic) && c isa Number
-                if isinteger(c) && !iszero(c)
+                if (c == 1 || c == -1) && !iszero(c)
                     c = convert(Integer, c)
                     linear_term += c * var
                     push!(coeffs, c)