// Arup Guha
// 4/13/2018
// Solution to 2018 Code Jam Round 1A Problem: Waffle Choppers
// Written in contest, commented later.

import java.util.*;

public class Solution {

	public static int r;
	public static int c;
	public static int h;
	public static int v;

	public static int numChips;

	public static int[][] grid;

	public static void main(String[] args) {

		Scanner stdin = new Scanner(System.in);
		int nC = stdin.nextInt();

		// Process each case.
		for (int loop=1; loop<=nC; loop++) {

			r = stdin.nextInt();
			c = stdin.nextInt();
			grid = new int[r][c];
			h = stdin.nextInt();
			v = stdin.nextInt();

			// Read in the grid.
			numChips = 0;
			for (int i=0; i<r; i++) {
				String s = stdin.next();
				for (int j=0; j<c; j++) {
					if (s.charAt(j) == '.')
						grid[i][j] = 0;
					else {
						grid[i][j] = 1;
						numChips++;
					}
				}
			}

			// Solve and output the result.
			boolean res = solve();
			if (res)
				System.out.println("Case #"+loop+": POSSIBLE");
			else
				System.out.println("Case #"+loop+": IMPOSSIBLE");
		}
	}

	public static boolean solve() {

		// A number of items we can check via basic divisibility.
		if (numChips%(h+1) != 0) return false;
		if (numChips%(v+1) != 0) return false;
		if (numChips%((h+1)*(v+1)) != 0) return false;
		if (numChips == 0) return true;

		ArrayList<Integer> hCuts = new ArrayList<Integer>();

		// How many chips we need after cuts all in one direction.
		int target = numChips/(h+1);
		int cur = 0;
		
		// We just greedily cut as soon as we can for horizontal cuts.
		for (int i=0; i<r; i++) {

			for (int j=0;j<c;j++)
				cur += grid[i][j];

			// Time to cut.
			if (cur == target) {
				hCuts.add(i);
				cur = 0;
			}
			
			// If this triggers, it means we went from too few to too many, so it's impossible.
			else if (cur > target)
				return false;
		}

		// New target in a single piece.
		int cTarget = numChips/((h+1)*(v+1));

		int[] pCur = new int[h+1];

		// Go column by column to see if we can cut.
		for (int myc=0; myc<c; myc++) {

			// Across each slice we add the chocolate chips, just for this single column for each piece.
			for (int i=0; i<hCuts.size(); i++) {
				int start = i == 0 ? 0 : hCuts.get(i-1)+1;
				int end = hCuts.get(i);
				for (int j=start; j<=end; j++) {
					pCur[i] += grid[j][myc];
				}
			}

			// We check if every piece has the correct number of pieces.
			// If any piece has gone over, it's impossible.
			boolean canCut = true;
			for (int i=0; i<hCuts.size(); i++) {
				if (pCur[i] != cTarget) canCut = false;
				if (pCur[i] > cTarget) return false;
			}

			// This means we can make a vertical cut so all pieces to its left are correct.
			// Just reset our counters to 0.
			if (canCut) {
				Arrays.fill(pCur, 0);
			}
		}

		return true;
	}
}