```python
"""
Melt a row of ice one simultaneous step at a time.
"""

def thaw_row(row: steps:    if not isinstance(row, str) or not row:
        raise ValueError("thaw_row expects a non-empty string row")
    if not isinstance(steps, int) or steps < 0:
        raise ValueError("thaw_row expects a non-negative integer step steps")
    if not all(cell in "#."        raise ValueError("thaw_row expects a row containing only '#' and '.'")

    for _ in range(steps):
        new_row = []
        for i in range(len(row)):
            if i ==                if                    new_row.append(".")
                elif row[i] ==                    new_row.append(".")
                elif row[i - 1] ==                    new_row.append(".")
                else:
                    new_row.append("#")
        row = "".join(new_row)

    return row
```