diff --git a/src/rulenode_operators.jl b/src/rulenode_operators.jl
index 5c8cc14..b5ef545 100644
--- a/src/rulenode_operators.jl
+++ b/src/rulenode_operators.jl
@@ -293,12 +293,48 @@ nchildren(grammar::Grammar, node::RuleNode)::Int = length(child_types(grammar, n
 """
 	isvariable(grammar::Grammar, node::RuleNode)::Bool
 
-Returns true if the rule used by `node` represents a variable.
+Return true if the rule used by `node` represents a variable in a program (essentially, an input to the program)
 """
-isvariable(grammar::Grammar, node::RuleNode)::Bool = grammar.isterminal[node.ind] && grammar.rules[node.ind] isa Symbol
+isvariable(grammar::Grammar, node::RuleNode)::Bool = (
+    grammar.isterminal[node.ind] &&
+    grammar.rules[node.ind] isa Symbol &&
+    !_is_defined_in_modules(grammar.rules[node.ind], [Main, Base])
+)
+"""
+	isvariable(grammar::Grammar, node::RuleNode, mod::Module)::Bool
+
+Return true if the rule used by `node` represents a variable.
+	
+Taking into account the symbols defined in the given module(s).
+"""
+isvariable(grammar::Grammar, node::RuleNode, mod::Module...)::Bool = (
+	grammar.isterminal[node.ind] &&
+	grammar.rules[node.ind] isa Symbol &&
+	!_is_defined_in_modules(grammar.rules[node.ind], [mod..., Main, Base])
+)
+
+"""
+	isvariable(grammar::Grammar, ind::Int)::Bool
 
-isvariable(grammar::Grammar, ind::Int)::Bool = grammar.isterminal[ind] && grammar.rules[ind] isa Symbol
+Return true if the rule with index `ind` represents a variable.
+"""
+isvariable(grammar::Grammar, ind::Int)::Bool = (
+	grammar.isterminal[ind] &&
+	grammar.rules[ind] isa Symbol &&
+	!_is_defined_in_modules(grammar.rules[ind], [Main, Base])
+)
+"""
+	isvariable(grammar::Grammar, ind::Int, mod::Module)::Bool
 
+Return true if the rule with index `ind` represents a variable.
+	
+Taking into account the symbols defined in the given module(s).
+"""
+isvariable(grammar::Grammar, ind::Int, mod::Module...)::Bool = (
+	grammar.isterminal[ind] &&
+	grammar.rules[ind] isa Symbol &&
+	!_is_defined_in_modules(grammar.rules[ind], [mod..., Main, Base])
+)
 
 """
 	contains_returntype(node::RuleNode, grammar::Grammar, sym::Symbol, maxdepth::Int=typemax(Int))
diff --git a/src/utils.jl b/src/utils.jl
index 76a937d..e1f7ca6 100644
--- a/src/utils.jl
+++ b/src/utils.jl
@@ -78,20 +78,24 @@ function _add_to_symboltable!(tab::SymbolTable, rule::Expr, mod::Module)
     return true
 end
 
+function _apply_if_defined_in_modules(func::Function, s::Symbol, mods::Vector{Module})
+    for mod in mods
+        if isdefined(mod, s)
+            func(mod, s)
+            return true
+        end
+    end
+    return false
+end
+
+function _is_defined_in_modules(s::Symbol, mods::Vector{Module})
+    _apply_if_defined_in_modules((mod, s) -> nothing, s, mods)
+end
 
 function _add_to_symboltable!(tab::SymbolTable, s::Symbol, mod::Module)
-    if isdefined(mod, s)
-        tab[s] = getfield(mod, s)
-        return true
-    elseif isdefined(Base, s)
-        tab[s] = getfield(Base, s)
-        return true
-    elseif isdefined(Main, s)
-        tab[s] = getfield(Main, s)
-        return true
-    else
-        return false
-    end
+    _add_to_table! = (mod, s) -> tab[s] = getfield(mod, s)
+
+    return _apply_if_defined_in_modules(_add_to_table!, s, [mod, Base, Main])
 end
 
 
diff --git a/test/runtests.jl b/test/runtests.jl
index b64a4c3..60de23b 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -3,4 +3,5 @@ using Test
 
 @testset "HerbGrammar.jl" verbose=true begin
     include("test_cfg.jl")
+    include("test_rulenode_operators.jl")
 end
diff --git a/test/test_rulenode_operators.jl b/test/test_rulenode_operators.jl
new file mode 100644
index 0000000..dee0c14
--- /dev/null
+++ b/test/test_rulenode_operators.jl
@@ -0,0 +1,18 @@
+module SomeDefinitions
+    a_variable_that_is_defined = 7
+end
+
+@testset verbose = true "RuleNode Operators" begin
+    @testset "Check if a symbol is a variable" begin
+        g₁ = @cfgrammar begin
+            Real = |(1:5)
+            Real = a_variable
+            Real = a_variable_that_is_defined
+        end
+
+        @test !isvariable(g₁, RuleNode(5, g₁), SomeDefinitions)
+        @test isvariable(g₁, RuleNode(6, g₁), SomeDefinitions)
+        @test !isvariable(g₁, RuleNode(7, g₁), SomeDefinitions)
+        @test isvariable(g₁, RuleNode(7, g₁))
+    end
+end