code: function validateVar({ spec = {}, name, rawValue }) {
      if (typeof spec._parse !== 'function') {
          throw new EnvError(`Invalid spec for "${name}"`)
      }
      const value = spec._parse(rawValue)
  
      if (spec.choices) {
          if (!Array.isArray(spec.choices)) {
              throw new TypeError(`"choices" must be an array (in spec for "${name}")`)
          } else if (!spec.choices.includes(value)) {
              throw new EnvError(`Value "${value}" not in choices [${spec.choices}]`)
          }
      }
      if (value == null) throw new EnvError(`Invalid value for env var "${name}"`)
      return value
  }
docs: Validate a single env var, given a spec object
  
  @throws EnvError - If validation is unsuccessful
  @return - The cleaned value