-- An expression to evaluate
local expr = 'a + (b or 0)'

-- Set up two different environments we want our expression to be evaluated against
local env1,env2 = {a=17}, {a=1,b=25}

-- Create a wrapper environment with a metatable
local envmeta = {}
local envwrap = setmetatable({},envmeta)

-- Load the expression into a function
local f = load('return '..expr, nil, nil, envwrap)

-- Evaluate the expression in the context of env1
envmeta.__index = env1
print(f()) --> 17

-- Evaluate the expression in the context of env2
envmeta.__index = env2
print(f()) --> 26

-- You can even combine environments, with env1 shadowing env2
envmeta.__index = env1
setmetatable(env1,{__index=env2})
print(f()) --> 42