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+Here's an example of an LPegLabel grammar that make its own function called
+'expect', which takes a pattern and a label as parameters and throws the label
+if the pattern fails to be matched. This function can be extended later on to
+record all errors encountered once error recovery is implemented.
+
+```lua
+local lpeg = require"lpeglabel"
+
+local R, S, P, V, C, Ct, T = lpeg.R, lpeg.S, lpeg.P, lpeg.V, lpeg.C, lpeg.Ct, lpeg.T
+
+local labels = {
+  {"NoExp",     "no expression found"},
+  {"Extra",     "extra characters found after the expression"},
+  {"ExpTerm",   "expected a term after the operator"},
+  {"ExpExp",    "expected an expression after the parenthesis"},
+  {"MisClose",  "missing a closing ')' after the expression"},
+}
+
+local function expect(patt, labname)
+  for i, elem in ipairs(labels) do
+    if elem[1] == labname then
+      return patt + T(i)
+    end
+  end
+
+  error("could not find label: " .. labname)
+end
+
+local num = R("09")^1 / tonumber
+local op = S("+-*/")
+
+local function compute(tokens)
+  local result = tokens[1]
+  for i = 2, #tokens, 2 do
+    if tokens[i] == '+' then
+      result = result + tokens[i+1]
+    elseif tokens[i] == '-' then
+      result = result - tokens[i+1]
+    elseif tokens[i] == '*' then
+      result = result * tokens[i+1]
+    elseif tokens[i] == '/' then
+      result = result / tokens[i+1]
+    else
+      error('unknown operation: ' .. tokens[i])
+    end
+  end
+  return result
+end
+
+local g = P {
+  "Exp",
+  Exp = Ct(V"Term" * (C(op) * expect(V"Term", "ExpTerm"))^0) / compute;
+  Term = num + V"Group";
+  Group = "(" * expect(V"Exp", "ExpExp") * expect(")", "MisClose");
+}
+
+g = expect(g, "NoExp") * expect(-P(1), "Extra")
+
+local function eval(input)
+  local result, label, suffix = g:match(input)
+  if result ~= nil then
+    return result
+  else
+    local pos = input:len() - suffix:len() + 1
+    local msg = labels[label][2]
+    return nil, "syntax error: " .. msg .. " (at index " .. pos .. ")"
+  end
+end
+
+print(eval "98-76*(54/32)")
+--> 37.125
+
+print(eval "(1+1-1*2/2")
+--> syntax error: missing a closing ')' after the expression (at index 11)
+
+print(eval "(1+)-1*(2/2)")
+--> syntax error: expected a term after the operator (at index 4)
+
+print(eval "(1+1)-1*(/2)")
+--> syntax error: expected an expression after the parenthesis (at index 10)
+
+print(eval "1+(1-(1*2))/2x")
+--> syntax error: extra chracters found after the expression (at index 14)
+
+print(eval "-1+(1-(1*2))/2")
+--> syntax error: no expression found (at index 1)
+```