Question: PPN is the abbreviation for what?
About the article "Nebula", section "Protoplanetary nebula".

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As the name indicates, the PPN confederated in 1946 with a grouping of regional of pro-independence parties within French West Africa and French Equatorial Africa to form the "Rassemblement Démocratique Africain" (African Democratic Rally—RDA ).
Under the leadership of Hamani Diori, the PPN paired appeals to traditional society within what was then the Colony of Niger while its representatives worked with the French Communist Party in France, which was then the only force supporting independence for France's colonial possessions.
In 1946 the single pro-Independence party in Niger was the Nigerien Progressive Party (PPN), of which Hamani Diori and Djibo Bakary were the most prominent members, and which numbered only 5000 members.
Simultaneously, these colonial territories were allowed limited representation in the French National Assembly, with Niger allotted one seat in 1946 and a second in 1948.
PPN Party leader Hamani Diori filled the first, and a French educated Niamey lawyer, Djibo Bakary filled the second.
Bakary, a leftist, helped push the party—already perceived as anti-French—in a populist direction.
The PPN was allied to the pan-colony African Democratic Rally (RDA), which itself caucused with the French Communist Party in the National Assembly.
Some elements, such as RDA leader Félix Houphouët-Boigny, were uncomfortable with this connection.
Many in the PPN felt the same way, while many other, grouped around Bakary and the tiny Nigerien Trades Union movement, pulled to the left.
Earlier splits of the PPN, of conservative Djerma traditional leaders and a small Franco-Nigerien contingent in 1946, were added to 1948 in reaction to Bakary and his circle and to the continued association with the RDA.
Harou Kouka and Georges Condat split to form a group (""Parti Independent du Niger-Est" PINE") that quickly joined with previous dissidents to create the Union of Nigerien Independents and Sympathizers (UNIS).
This relatively conservative coalition benefited from French support, and gained control of the consultative institutions of the Niger colony from 1948-1952.
Diori, much like RDA chair Félix Houphouët-Boigny, broke from the PCF in 1951 and by 1958 was close to the colonial authorities in Niamey.
This faction retained both the PPN name and the connection with the RDA, while Djibo Bakary now broke from the RDA to retain ties with the French Communist Party.
Bakary's new party, SAWABA, prospered in the 1956 territorial assembly elections to the detriment of the PPN.
In 1958's constitutional referendum and assembly elections, the PPN supported continued association with France under the French Community, while its primary rival, called for immediate independence from France.
The PPN swept territorial elections making Diori chair of the Assembly (essentially, Prime Minister to the colonial governor's presidential role).
SAWABA was suppressed and outlawed prior to independence in 1960 with the help of French officials. 
  AN/PPN-1  AN/PPN-1A  AN/PPN-2 American.
The "Rebecca" code name was derived from the phrase "recognition of beacons". 
In the laboratory, PPN chloride is the main precursor to PPN+ salts.
Using salt metathesis reactions, nitrite, azide, and other small inorganic anions can be obtained with PPN+ cations.
The resulting salts PPNNO2, PPNN3 etc.
are soluble in polar organic solvents.PPN+ forms crystalline salts with a range of anions that are otherwise difficult to crystallize.
Its effectiveness is partially attributable to its rigidity, reflecting the presence of six phenyl rings.
Often PPN+ forms salts that are more air-stable than salts with smaller cations such as those containing quaternary ammonium or alkali metal cations.
This effect is attributed to the steric shielding provided by this voluminous cation.
Illustrative PPN+ salts of reactive anions include PPNPPN[Co(CO)4, and PPN[Fe(CO)3NO].
The role of ion pairing in chemical reactions is often clarified by examination of the related salt derived from PPN+. 
Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula [(C6H5)3P)2N]Cl, often written [(Ph3P)2N]Cl and abbreviated PPNCl.
This colorless salt is a source of the PPN+ cation, which is used to isolate reactive anions.
PPN+ is a phosphazene. 
The pedunculopontine nucleus (PPN) is a part of the brain that controls the planning and execution of gait.
The PPN is connected extensively with other parts of the brain, including the spinal cord, cortex, and basal ganglia, and these regions work together to plan, initiate, and maintain gait.