FILTER_CONV : conv -> conv
FILTER P [x0;...xn]
|- FILTER P [x0;...xn] = [...xi...]
FILTER_CONV bool_EQ_CONV (--`FILTER ($= T) [T;F;T]`--);
|- FILTER($= T)[T;F;T] = [T;T]
(BETA_CONV THENC conv')