from typing import List

def matmul(A: List[List[float]], B: List[List[float]]) -> List[List[float]]:
    """
    Multiplies two matrices A and B (nested Python lists).

    Requirements:
      - A has shape (m x n), B has shape (n x p).
      - All rows in A and B must have equal length.
      - Elements are numeric (float/int).

    Args:
        A: Left matrix, list of rows (m x n).
        B: Right matrix, list of rows (n x p).

    Returns:
        New matrix C = A * B with shape (m x p).

    Raises:
        ValueError: If matrices are empty, ragged,
                    or shapes are incompatible.

    """
    if not A or not B:
        raise ValueError("Empty matrices are not supported.")
    if not A[0] or not B[0]:
        raise ValueError("Matrices must have at least one column.")

    a_cols = len(A[0])

    for row in A:
        if len(row) != a_cols:
            raise ValueError("Left matrix has inconsistent row lengths.")
    b_cols = len(B[0])
    for row in B:
        if len(row) != b_cols:
            raise ValueError("Right matrix has inconsistent row lengths.")

    if a_cols != len(B):
        raise ValueError(
            f"Incompatible shapes: A is {len(A)}x{a_cols}, "
            f"B is {len(B)}x{b_cols}; need cols(A) == rows(B)."
        )

    m, n, p = len(A), a_cols, b_cols
    C: List[List[float]] = [[0 for _ in range(p)] for _ in range(m)]

    for i in range(m):
        for k in range(n):
            a_ik = A[i][k]
            for j in range(p):
                C[i][j] += a_ik * B[k][j]

    return C


if __name__ == "__main__":
    matrix_a = [[3, 4, -1, 4],
                [-2, 2, 5, 1]]
    matrix_b = [[1, 3, -2],
                [2, 5, 1],
                [-1, 4, -4],
                [2, 3, 6]]
    matrix_c = matmul(matrix_a, matrix_b)
    print("Ergebnis C = A * B:")
    for row in matrix_c:
        print(row)