JMO 1992 Problem 2

Suppose that $D,\ E$ are points on $AB,\ AC$ of $\triangle{ABC}$ with area $1$ respectively and $P$ is $BE\cap CD$. When $D,\ E$ move on $AB,\ AC$ with satisfying the condition $[BCED] = 2\triangle{PBC}$, find the maximum area of $\triangle{PDE}$.