def replace_first_sentence(text, replacement):
    """ Replace the first sentence in text with replacement. This makes
    some incredibly simplifying assumptions - so buyer beware. """
    no_periods_replacement = replacement.replace('.', '')
    sentences = text.split('.', 1)
    if len(sentences) > 1:
        sentences[0] = no_periods_replacement
        return '.'.join(sentences)
    else:
        return replacement
