[ BeginEnv "proposition"
, Label "sep_test"
, BeginEnv "math"
, Variable "X"
, Symbol "="
, VisibleBraceL
, Variable "x"
, Command "in"
, Variable "X"
, Command "mid"
, Variable "x"
, Symbol "="
, Variable "x"
, VisibleBraceR
, EndEnv "math"
, Symbol "."
, EndEnv "proposition"
]