code: def solve_let(expr, vars):
      
      lhs_value = solve(expr.lhs, vars).value
      if not isinstance(lhs_value, structured.IStructured):
          raise errors.EfilterTypeError(
              root=expr.lhs, query=expr.original,
              message="The LHS of 'let' must evaluate to an IStructured. Got %r."
              % (lhs_value,))
  
      return solve(expr.rhs, __nest_scope(expr.lhs, vars, lhs_value))
docs: Solves a let-form by calling RHS with nested scope.