var blit = (function(){
	var blit = {}
	blit.and = blit.atop = function(A, B, x, y){
		x = x || 0 ; y = y || 0
		B.forEach(function(lex, u, v){
			var cell = A.getCell(u+x, v+y)
			if (cell && lex.opacity > 0) {
				cell.clone(lex)
			}
		})
	}
	blit.or = blit.under = function(A, B, x, y){
		x = x || 0 ; y = y || 0
		B.forEach(function(lex, u, v){
			var cell = A.getCell(u+x, v+y)
			if (cell && cell.opacity == 0) {
				cell.clone(lex)
			}
		})
	}
	// copy the region of A beginning at x,y into B
	blit.copy_from = function(A, B, x, y){
		x = x || 0 ; y = y || 0
		B.forEach(function(lex, u, v){
			var cell = A.getCell(u+x, v+y)
			if (cell) {
				lex.clone(cell)
			}
		})
	}
	blit.copy_to = function(A, B, x, y){
		x = x || 0 ; y = y || 0
		B.forEach(function(lex, u, v){
			var cell = A.getCell(u+x, v+y)
			if (cell) {
				cell.clone(lex)
			}
		})
	}
	blit.invert = function(A, B, x, y){
		x = x || 0 ; y = y || 0
		B.forEach(function(lex, u, v){
			var cell = A.getCell(u+x, v+y)
			if (cell && lex.opacity > 0) {
				cell.fg = get_inverse(cell.fg)
				cell.bg = get_inverse(cell.bg)
			}
		})
	}
	return blit
})()