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Given the following text problem statement, write Python code to implement the functionality described below in problem statement
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Problem:
I've seen similar questions but mine is more direct and abstract.
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Python Code:
import pandas as pd
df = pd.DataFrame([[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15]],columns=['A','B','C','D','E'])
def g(df):
    df.index += 1
    df_out = df.stack()
    df.index -= 1
    df_out.index = df_out.index.map('{0[1]}_{0[0]}'.format)
    return df_out.to_frame().T
df = g(df.copy())
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