diff --git a/src/Runic.jl b/src/Runic.jl
index b939dc4..1cdcb6b 100644
--- a/src/Runic.jl
+++ b/src/Runic.jl
@@ -5,7 +5,7 @@ module Runic
 using JuliaSyntax:
     JuliaSyntax, @K_str, @KSet_str
 
-# compat for const fields
+# Julia compat for const struct fields
 @eval macro $(Symbol("const"))(field)
     if VERSION >= v"1.8.0-DEV.1148"
         Expr(:const, esc(field))
@@ -14,7 +14,8 @@ using JuliaSyntax:
     end
 end
 
-include("chisel.jl")
+# JuliaSyntax extensions and other utilities
+include("chisels.jl")
 
 # Return the result of expr if it doesn't evaluate to `nothing`
 macro return_something(expr)
@@ -33,6 +34,7 @@ mutable struct Context
     fmt_tree::Union{JuliaSyntax.GreenNode{JuliaSyntax.SyntaxHead}, Nothing}
     # User settings
     verbose::Bool
+    assert::Bool
     debug::Bool
     # Current state
     # node::Union{JuliaSyntax.GreenNode{JuliaSyntax.SyntaxHead}, Nothing}
@@ -41,14 +43,17 @@ mutable struct Context
     # parent::Union{JuliaSyntax.GreenNode{JuliaSyntax.SyntaxHead}, Nothing}
 end
 
-function Context(src_str; debug::Bool = false, verbose::Bool = debug)
+function Context(src_str; assert::Bool = true, debug::Bool = false, verbose::Bool = debug)
     src_io = IOBuffer(src_str)
     src_tree = JuliaSyntax.parseall(JuliaSyntax.GreenNode, src_str; ignore_warnings = true)
     fmt_io = IOBuffer()
     fmt_tree = nothing
+    # Debug mode enforces verbose and assert
+    verbose = debug ? true : verbose
+    assert = debug ? true : assert
     return Context(
-        src_str, src_tree, src_io, fmt_io, fmt_tree, verbose, debug,
-        nothing, nothing,
+        src_str, src_tree, src_io, fmt_io, fmt_tree, verbose, assert, debug, nothing,
+        nothing,
     )
 end
 
diff --git a/src/ToggleableAsserts.jl b/src/ToggleableAsserts.jl
new file mode 100644
index 0000000..4928952
--- /dev/null
+++ b/src/ToggleableAsserts.jl
@@ -0,0 +1,44 @@
+# SPDX-License-Identifier: MIT
+
+# The code in this file is derived from the ToggleableAsserts.jl package
+# (https://github.com/MasonProtter/ToggleableAsserts.jl) licensed under the MIT license.
+# (https://github.com/MasonProtter/ToggleableAsserts.jl/blob/master/LICENSE):
+
+# MIT License
+#
+# Copyright (c) 2021 Mason Protter
+#
+# Permission is hereby granted, free of charge, to any person obtaining a copy
+# of this software and associated documentation files (the "Software"), to deal
+# in the Software without restriction, including without limitation the rights
+# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+# copies of the Software, and to permit persons to whom the Software is
+# furnished to do so, subject to the following conditions:
+#
+# The above copyright notice and this permission notice shall be included in all
+# copies or substantial portions of the Software.
+#
+# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+# SOFTWARE.
+
+assert_enabled() = true
+
+macro assert(expr)
+    code = macroexpand_assert(expr)
+    :(assert_enabled() ? $(code) : nothing)
+end
+
+const toggle_lock = ReentrantLock()
+
+function enable_assert(enable::Bool)
+    @lock toggle_lock begin
+        if assert_enabled() != enable
+            @eval Runic assert_enabled() = $enable
+        end
+    end
+end
diff --git a/src/chisel.jl b/src/chisels.jl
similarity index 58%
rename from src/chisel.jl
rename to src/chisels.jl
index c114947..1407214 100644
--- a/src/chisel.jl
+++ b/src/chisels.jl
@@ -1,5 +1,42 @@
 # SPDX-License-Identifier: MIT
 
+##############
+# Debug info #
+##############
+
+# @lock is defined but not exported in older Julia versions
+if VERSION < v"1.7.0"
+    using Base: @lock
+end
+
+# Code derived from ToggleableAsserts.jl kept in a separate file
+include("ToggleableAsserts.jl")
+
+abstract type RunicException <: Exception end
+
+struct AssertionError <: RunicException
+    msg::String
+end
+
+function Base.showerror(io::IO, err::AssertionError)
+    print(
+        io,
+        "Runic.AssertionError: `", err.msg, "`. This is unexpected, " *
+        "please file an issue with a reproducible example at " *
+        "https://github.com/fredrikekre/Runic.jl/issues/new.",
+    )
+end
+
+function macroexpand_assert(expr)
+    msg = string(expr)
+    return :($(esc(expr)) || throw(AssertionError($msg)))
+end
+
+
+##########################
+# JuliaSyntax extensions #
+##########################
+
 function is_leaf(node::JuliaSyntax.GreenNode)
     return !JuliaSyntax.haschildren(node)
 end
@@ -23,10 +60,13 @@ end
 function is_assignment(node::JuliaSyntax.GreenNode)
     return JuliaSyntax.is_prec_assignment(node)
 end
+
+# Just like `JuliaSyntax.is_infix_op_call`, but also check that the node is K"call"
 function is_infix_op_call(node::JuliaSyntax.GreenNode)
     return JuliaSyntax.kind(node) === K"call" &&
         JuliaSyntax.is_infix_op_call(node)
 end
+
 function infix_op_call_op(node::JuliaSyntax.GreenNode)
     @assert is_infix_op_call(node)
     children = JuliaSyntax.children(node)::AbstractVector
@@ -34,9 +74,11 @@ function infix_op_call_op(node::JuliaSyntax.GreenNode)
     op_index = findnext(JuliaSyntax.is_operator, children, first_operand_index + 1)
     return children[op_index]
 end
+
 function is_comparison_leaf(node::JuliaSyntax.GreenNode)
     return is_leaf(node) && JuliaSyntax.is_prec_comparison(node)
 end
+
 function is_operator_leaf(node::JuliaSyntax.GreenNode)
     return is_leaf(node) && JuliaSyntax.is_operator(node)
 end